Light-emitting element, light-emitting device, display device, electronic device, and lighting device

ABSTRACT

An object is to provide a light-emitting element which uses a plurality of kinds of light-emitting dopants and has high emission efficiency. In one embodiment of the present invention, a light-emitting device, a light-emitting module, a light-emitting display device, an electronic device, and a lighting device each having reduced power consumption by using the above light-emitting element are provided. Attention is paid to Förster mechanism, which is one of mechanisms of intermolecular energy transfer. Efficient energy transfer by Förster mechanism is achieved by making an emission wavelength of a molecule which donates energy overlap with the longest-wavelength-side local maximum peak of a graph obtained by multiplying an absorption spectrum of a molecule which receives energy by a wavelength raised to the fourth power.

TECHNICAL FIELD

The present invention relates to a light-emitting element, a displaydevice, a light-emitting device, an electronic device, and a lightingdevice each of which uses an organic compound as a light-emittingsubstance.

BACKGROUND ART

In recent years, research and development have been extensivelyconducted on light-emitting elements utilizing electroluminescence (EL).In the basic structure of such a light-emitting element, a layercontaining a light-emitting substance (an EL layer) is interposedbetween a pair of electrodes. By voltage application to this element,light emission from the light-emitting substance can be obtained.

Such light-emitting elements are self-luminous elements and haveadvantages over liquid crystal displays in having high pixel visibilityand eliminating the need for backlights, for example; thus, suchlight-emitting elements are thought to be suitable for flat paneldisplay elements. Displays including such light-emitting elements arealso highly advantageous in that they can be thin and lightweight.Furthermore, very high speed response is one of the features of suchelements.

Since light-emitting layers of such light-emitting elements can beformed in a film form, they make it possible to provide planar lightemission. Therefore, large-area elements can be easily formed. This is afeature difficult to obtain with point light sources typified byincandescent lamps and LEDs or linear light sources typified byfluorescent lamps. Thus, the light-emitting elements also have greatpotential as planar light sources applicable to lightings and the like.

In the case of an organic EL element in which an organic compound isused as a light-emitting substance and the EL layer is provided betweena pair of electrodes, application of a voltage between the pair ofelectrodes causes injection of electrons from a cathode and holes froman anode into the EL layer having a light-emitting property and thus acurrent flows. By recombination of the injected electrons and holes, theorganic compound having a light-emitting property is put in an excitedstate to provide light emission.

It is to be noted that the excited states formed by an organic compoundinclude a singlet excited state and a triplet excited state, andluminescence from the singlet excited state (S*) is referred to asfluorescence, whereas luminescence from the triplet excited state (T*)is referred to as phosphorescence. In addition, the statisticalgeneration ratio thereof in the light-emitting element is considered tobe as follows: S*:T*=1:3.

In a compound that emits light from the singlet excited state(hereinafter, referred to as a fluorescent compound), at roomtemperature, generally light emission from the triplet excited state(phosphorescence) is not observed while only light emission from thesinglet excited state (fluorescence) is observed. Therefore, theinternal quantum efficiency (the ratio of generated photons to injectedcarriers) of a light-emitting element using a fluorescent compound isassumed to have a theoretical limit of 25% based on the ratio of S* toT* which is 1:3.

In contrast, in a compound that emits light from the triplet excitedstate (hereinafter, referred to as a phosphorescent compound), lightemission from the triplet excited state (phosphorescence) is observed.Further, in a phosphorescent compound, since intersystem crossing (i.e.,transfer from a singlet excited state to a triplet excited state) easilyoccurs, the internal quantum efficiency can be increased to 100% intheory. That is, higher emission efficiency can be achieved than using afluorescent compound. For this reason, light-emitting elements usingphosphorescent compounds are now under active development in order toobtain highly efficient light-emitting elements.

A white light-emitting element disclosed in Patent Document 1 includes alight-emitting region containing a plurality of kinds of light-emittingdopants which emit phosphorescence.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Translation of PCT International    Application No. 2004-522276

DISCLOSURE OF INVENTION

Although an internal quantum efficiency of 100% in a phosphorescentcompound is theoretically possible, such high efficiency can be hardlyachieved without optimization of an element structure or a combinationwith another material. Especially in a light-emitting element whichincludes a plurality of kinds of phosphorescent compounds havingdifferent bands (different emission colors) as light-emitting dopants,it is difficult to obtain highly efficient light emission without notonly considering energy transfer but also optimizing the efficiency ofthe energy transfer. In fact, in Patent Document 1, even when all thelight-emitting dopants of a light-emitting element are phosphorescentcompounds, the external quantum efficiency is approximately 3% to 4%. Itis thus presumed that even when light extraction efficiency is takeninto account, the internal quantum efficiency is 20% or lower, which islow for a phosphorescent light-emitting element.

In a multicolor light-emitting element using dopants exhibitingdifferent emission colors, beside improvement of emission efficiency, itis also necessary to attain a good balance between light emissions bythe dopants which exhibit different emission colors. It is not easy tokeep a balance between light emissions by the dopants and to achievehigh emission efficiency at the same time.

In view of the above, an object of one embodiment of the presentinvention is to provide a light-emitting element which uses a pluralityof kinds of light-emitting dopants and has high emission efficiency.Another object of one embodiment of the present invention is to providea light-emitting device, a display device, an electronic device, and alighting device each having reduced power consumption by using the abovelight-emitting element.

It is only necessary that at least one of the above objects be achievedin the present invention.

In one embodiment of the present invention, focus is placed on Förstermechanism, which is one of mechanisms of intermolecular energy transfer,and efficient energy transfer by Förster mechanism is achieved byemploying a combination of molecules which makes it possible to obtainan overlap between an emission spectrum band of the molecule whichdonates energy and the longest-wavelength-side peak of a characteristiccurve obtained by multiplying an absorption spectrum of the moleculewhich receives energy by a wavelength raised to the fourth power. Here,one of the characteristics of the above energy transfer is that theenergy transfer is not general energy transfer from a host to a dopantbut energy transfer from a dopant to a dopant. The light-emittingelement of one embodiment of the present invention can be obtained byemploying such a combination of dopants between which energy can betransferred so efficiently and designing an element structure such thatdopant molecules are appropriately isolated.

That is, one embodiment of the present invention is a light-emittingelement including, between a pair of electrodes, a first light-emittinglayer in which a first phosphorescent compound is dispersed in a firsthost material; and a second light-emitting layer in which a secondphosphorescent compound emitting light with a wavelength longer thanthat of light emitted from the first phosphorescent compound isdispersed in a second host material. A wavelength of thelongest-wavelength-side peak of a function ε(λ)λ⁴ of the secondphosphorescent compound overlaps with a phosphorescence spectrum F(λ) ofthe first phosphorescent compound. Note that ε(λ) denotes a molarabsorption coefficient of each of the phosphorescent compounds and is afunction of a wavelength λ.

Another embodiment of the present invention is a light-emitting elementincluding, between a pair of electrodes, a first light-emitting layer inwhich a first phosphorescent compound is dispersed in a first hostmaterial; and a second light-emitting layer in which a secondphosphorescent compound emitting light with a wavelength longer thanthat of light emitted from the first phosphorescent compound isdispersed in a second host material. A band having the peak of aphosphorescence spectrum of the first phosphorescent compound overlapswith a band having the longest-wavelength-side peak of a function ε(λ)λ⁴of the second phosphorescent compound. Note that ε(λ) denotes a molarabsorption coefficient of each of the phosphorescent compounds and is afunction of a wavelength λ.

A further embodiment of the present invention is a light-emittingelement having the above structure in which the first light-emittinglayer further contains a first organic compound, the first host materialand the first organic compound form an exciplex, and light emitted fromthe first phosphorescent compound has a longer wavelength than lightemitted from the exciplex.

A still further embodiment of the present invention is a light-emittingelement having the above structure in which an emission spectrum of theexciplex overlaps with a wavelength of the longest-wavelength-side peakof a function ε(λ)λ⁴ of the first phosphorescent compound. Note thatε(λ) denotes a molar absorption coefficient of each of thephosphorescent compounds and is a function of a wavelength λ.

A yet still further embodiment of the present invention is alight-emitting element having the above structure in which a band havingthe peak of the emission spectrum of the exciplex overlaps with a bandhaving the longest-wavelength-side peak of the function ε(λ)λ⁴ of thefirst phosphorescent compound. Note that ε(λ) denotes a molar absorptioncoefficient of each of the phosphorescent compounds and is a function ofa wavelength λ.

A yet still further embodiment of the present invention is alight-emitting element having the above structure in which the firstphosphorescent compound has a phosphorescence peak in a range of 500 nmto 600 nm, and the second phosphorescent compound has a phosphorescencepeak in a range of 600 nm to 700 nm.

A yet still further embodiment of the present invention is alight-emitting element having the above structure in which arecombination region of an electron and a hole is the firstlight-emitting layer.

A yet still further embodiment of the present invention is alight-emitting element having the above structure in which the firstlight-emitting layer is positioned closer to an anode than the secondlight-emitting layer, and an electron-transport property is higher thana hole-transport property at least in the second light-emitting layer.

A yet still further embodiment of the present invention is alight-emitting element having the above structure in which the firstlight-emitting layer is positioned closer to the anode than the secondlight-emitting layer, and both the first host material and the secondhost material have an electron-transport property. Note that a materialhaving an electron-transport property is preferably a material in whichan electron-transport property is higher than a hole-transport property.

A yet still further embodiment of the present invention is alight-emitting element having the above structure in which the firstlight-emitting layer is positioned closer to a cathode than the secondlight-emitting layer, and a hole-transport property is higher than anelectron-transport property at least in the second light-emitting layer.

A yet still further embodiment of the present invention is alight-emitting element having the above structure in which the firstlight-emitting layer is positioned closer to the cathode than the secondlight-emitting layer, and both the first host material and the secondhost material have a hole-transport property. Note that a materialhaving a hole-transport property is preferably a material in which ahole-transport property is higher than an electron-transport property.

A yet still further embodiment of the present invention is alight-emitting element having the above structure in which the firstlight-emitting layer and the second light-emitting layer are stacked incontact with each other.

A yet still further embodiment of the present invention is alight-emitting device, a light-emitting display device, an electronicdevice, and a lighting device each including a light-emitting elementhaving the above structure.

Note that the light-emitting device in this specification includes, inits category, an image display device using a light-emitting element.Further, the category of the light-emitting device in this specificationincludes a module in which a light-emitting element is provided with aconnector such as an anisotropic conductive film or a TCP (tape carrierpackage); a module in which the top of the TCP is provided with aprinted wiring board; and a module in which an IC (integrated circuit)is directly mounted on a light-emitting element by a COG (chip on glass)method. Furthermore, the category includes light-emitting devices thatare used in lighting equipment or the like.

One embodiment of the present invention provides a light-emittingelement having high emission efficiency. By using the light-emittingelement, another embodiment of the present invention provides alight-emitting device, a light-emitting display device, an electronicdevice, and a lighting device each having reduced power consumption.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are conceptual diagrams of light-emitting elements.

FIGS. 2A and 2B illustrate energy transfer in light-emitting layers.

FIGS. 3A and 3B explain Förster energy transfer.

FIGS. 4A and 4B are conceptual diagrams of an active matrixlight-emitting device.

FIGS. 5A and 5B are conceptual diagrams of a passive matrixlight-emitting device.

FIGS. 6A and 6B are conceptual diagrams of structures of an activematrix light-emitting device.

FIG. 7 is a conceptual diagram of an active matrix light-emittingdevice.

FIGS. 8A and 8B are conceptual diagrams of a lighting device.

FIGS. 9A, 9B1, 9B2, 9C, and 9D each illustrate an electronic device.

FIG. 10 illustrates an electronic device.

FIG. 11 illustrates a lighting device.

FIG. 12 illustrates a lighting device and a display device.

FIG. 13 illustrates car-mounted display devices and lighting devices.

FIGS. 14A to 14C illustrate an electronic device.

FIG. 15 shows luminance-current efficiency characteristics of alight-emitting element 1.

FIG. 16 shows voltage-luminance characteristics of a light-emittingelement 1.

FIG. 17 shows luminance-external quantum efficiency characteristics of alight-emitting element 1.

FIG. 18 shows luminance-power efficiency characteristics of alight-emitting element 1.

FIG. 19 shows an emission spectrum of a light-emitting element 1.

FIGS. 20A and 20B explain Förster energy transfer in a light-emittingelement 1.

FIGS. 21A and 21B explain Förster energy transfer in a light-emittingelement 1.

FIG. 22 explains Förster energy transfer in a light-emitting element 1.

FIG. 23 shows PL spectra of 2mDBTPDBq-II, PCBA1BP, and a mixed filmthereof.

FIG. 24 shows luminance-current efficiency characteristics of alight-emitting element 2.

FIG. 25 shows voltage-luminance characteristics of a light-emittingelement 2.

FIG. 26 shows luminance-external quantum efficiency characteristics of alight-emitting element 2.

FIG. 27 shows luminance-power efficiency characteristics of alight-emitting element 2.

FIG. 28 shows an emission spectrum of a light-emitting element 2.

FIG. 29 shows a result of a reliability test of a light-emitting element2.

FIG. 30 shows luminance-current efficiency characteristics of alight-emitting element 3.

FIG. 31 shows voltage-luminance characteristics of a light-emittingelement 3.

FIG. 32 shows luminance-external quantum efficiency characteristics of alight-emitting element 3.

FIG. 33 shows luminance-power efficiency characteristics of alight-emitting element 3.

FIG. 34 shows an emission spectrum of a light-emitting element 3.

FIG. 35 shows luminance-current efficiency characteristics of alight-emitting element 4.

FIG. 36 shows voltage-luminance characteristics of a light-emittingelement 4.

FIG. 37 shows luminance-external quantum efficiency characteristics of alight-emitting element 4.

FIG. 38 shows luminance-power efficiency characteristics of alight-emitting element 4.

FIG. 39 shows an emission spectrum of a light-emitting element 4.

FIG. 40 shows luminance-current efficiency characteristics of alight-emitting element 5.

FIG. 41 shows voltage-luminance characteristics of a light-emittingelement 5.

FIG. 42 shows luminance-external quantum efficiency characteristics of alight-emitting element 5.

FIG. 43 shows luminance-power efficiency characteristics of alight-emitting element 5.

FIG. 44 shows an emission spectrum of a light-emitting element 5.

FIGS. 45A and 45B explain Förster energy transfer in a light-emittingelement 4.

FIG. 46 explains Förster energy transfer in a light-emitting element 4.

FIGS. 47A and 47B explain Förster energy transfer in a light-emittingelement 5.

FIGS. 48A and 48B explain Förster energy transfer in a light-emittingelement 5.

FIG. 49 explains Förster energy transfer in a light-emitting element 5.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings. Note that the present inventionis not limited to the description given below, and it will be easilyunderstood by those skilled in the art that various changes andmodifications can be made without departing from the spirit and scope ofthe present invention. Therefore, the present invention should not beinterpreted as being limited to the description of the embodiments givenbelow.

Embodiment 1

First, an operation principle of a light-emitting element of oneembodiment of the present invention will be described. The point of thepresent invention is that a first phosphorescent compound and a secondphosphorescent compound emitting light with a wavelength longer thanthat of light emitted from the first phosphorescent compound are usedand both of the first and second phosphorescent compounds are made toemit light efficiently, whereby a multicolor light-emitting element withhigh efficiency is obtained.

As a general method for obtaining a multicolor light-emitting elementincluding a phosphorescent compound, a method can be given in which aplurality of kinds of phosphorescent compounds having different emissioncolors are dispersed in some host material in an appropriate ratio.However, in such a method, the phosphorescent compound which emits lightwith the longest wavelength readily emits light, so that it is extremelydifficult to design and control a structure (especially theconcentrations of the phosphorescent compounds in the host material) forobtaining polychromatic light.

As another technique for obtaining a multicolor light-emitting element,what is called a tandem structure, in which light-emitting elementshaving different emission colors are stacked in series, can be given.For example, a blue light-emitting element, a green light-emittingelement, and a red light-emitting element are stacked in series and madeto emit light at the same time, whereby polychromatic light (in thiscase, white light) can be easily obtained. The element structure can berelatively easily designed and controlled because the bluelight-emitting element, the green light-emitting element, and the redlight-emitting element can be independently optimized. However, thestacking of three elements accompanies an increase in the number oflayers and makes the fabrication complicated. In addition, when aproblem occurs in electrical contact at connection portions between theelements (what is called intermediate layers), an increase in drivevoltage, i.e., power loss might be caused.

In contrast, in the light-emitting element of one embodiment of thepresent invention, between the pair of electrodes are stacked the firstlight-emitting layer in which the first phosphorescent compound isdispersed in the first host material and the second light-emitting layerin which the second phosphorescent compound emitting light with awavelength longer than that of light emitted from the firstphosphorescent compound is dispersed in the second host material. Here,unlike the case of a tandem structure, the first and secondlight-emitting layers may be provided in contact with each other.

An element structure of the above-described light-emitting element ofone embodiment of the present invention is schematically illustrated inFIGS. 1A to 1C. In FIG. 1C, a first electrode 101, a second electrode102, and an EL layer 103 are illustrated. The EL layer 103 includes atleast a light-emitting layer 113 and other layers may be provided asappropriate. In the structure illustrated in FIG. 1C, a hole-injectionlayer 111, a hole-transport layer 112, an electron-transport layer 114,and an electron-injection layer 115 are assumed to be provided. Notethat it is assumed that the first electrode 101 functions as an anodeand the second electrode 102 functions as a cathode.

FIGS. 1A and 1B are each an enlarged view of the light-emitting layer113 in the light-emitting element. In each of FIGS. 1A and 1B, a firstlight-emitting layer 113 a, a second light-emitting layer 113 b, thelight-emitting layer 113 which is a combination of the two layers, afirst phosphorescent compound 113Da, a second phosphorescent compound113Db, a first host material 113Ha, and a second host material 113Hb areillustrated. FIG. 1B is a schematic diagram illustrating the case wherethe first light-emitting layer 113 a further contains a first organiccompound 113A. In either case, the phosphorescent compounds (the firstand second phosphorescent compounds) are dispersed in the host materialsso that the phosphorescent compounds are isolated from each other by thehost materials. Note that the first and second host materials may be thesame or different from each other. The first light-emitting layer 113 amay be on the anode side and the second light-emitting layer 113 b maybe on the cathode side, or the first light-emitting layer 113 a may beon the cathode side and the second light-emitting layer 113 b may be onthe anode side.

In that case, between the phosphorescent compounds, energy transfer byelectron exchange interaction (what is called Dexter mechanism) issuppressed. In other words, a phenomenon in which after the firstphosphorescent compound 113Da is excited, the excitation energy istransferred to the second phosphorescent compound 113Db by Dextermechanism can be prevented. Thus, a phenomenon in which the secondphosphorescent compound 113Db emitting light with the longest wavelengthmainly emits light can be suppressed. Note that the secondphosphorescent compound 113Db mainly emits light in the case where anexciton is directly generated in the second light-emitting layer 113 b;therefore, it is preferable that a recombination region of carriers bein the first light-emitting layer 113 a (i.e., the first phosphorescentcompound 113Da be mainly excited).

Note that if energy transfer from the first phosphorescent compound113Da is completely suppressed, in turn, light emission from the secondphosphorescent compound 113Db cannot be obtained. Thus, in oneembodiment of the present invention, element design is performed suchthat excitation energy of the first phosphorescent compound 113Da ispartly transferred to the second phosphorescent compound 113Db. Suchenergy transfer between isolated molecules becomes possible by utilizingdipole-dipole interaction (Förster mechanism).

Here, Förster mechanism is described. The molecule which donatesexcitation energy and the molecule which receives excitation energy arehereinafter referred to as an energy donor and an energy acceptor,respectively. That is, in one embodiment of the present invention, boththe energy donor and the energy acceptor are phosphorescent compoundsand are isolated from each other by the host materials.

In Förster mechanism, direct intermolecular contact is not necessary forenergy transfer. Through a resonant phenomenon of dipolar oscillationbetween an energy donor and an energy acceptor, energy transfer occurs.The resonant phenomenon of dipolar oscillation causes the energy donorto donate energy to the energy acceptor; thus, the energy donor in anexcited state relaxes to a ground state and the energy acceptor in aground state is excited. The rate constant k_(F) of energy transfer byFörster mechanism is expressed by a formula (1).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack & \; \\{\mspace{175mu}{k_{F} = {\frac{9000\mspace{11mu} c^{4}K^{2}\phi\mspace{11mu}\ln\; 10}{128\;\pi^{5}n^{4}N\;\tau\; R^{6}}{\int{\frac{{F(v)}{ɛ(v)}}{v^{4}}{\mathbb{d}v}}}}}} & (1)\end{matrix}$

In the formula (1), ν denotes a frequency, F(ν) denotes a normalizedemission spectrum of an energy donor (a fluorescence spectrum in energytransfer from a singlet excited state, and a phosphorescence spectrum inenergy transfer from a triplet excited state), ε(ν) denotes a molarabsorption coefficient of an energy acceptor, N denotes Avogadro'snumber, n denotes a refractive index of a medium, R denotes anintermolecular distance between the energy donor and the energyacceptor, τ denotes a measured lifetime of an excited state(fluorescence lifetime or phosphorescence lifetime), c denotes the speedof light, φ denotes a luminescence quantum yield (a fluorescence quantumyield in energy transfer from a singlet excited state, and aphosphorescence quantum yield in energy transfer from a triplet excitedstate), and K² denotes a coefficient (0 to 4) of orientation of atransition dipole moment between the energy donor and the energyacceptor. Note that K²=⅔ in random orientation.

As the formula (1) suggests, the following can be given as necessaryconditions for energy transfer by Förster mechanism (Förster energytransfer): 1. the energy donor and the energy acceptor are not too farapart from each other (which relates to the distance R); 2. the energydonor emits light (which relates to the luminescence quantum yield φ);and 3. an emission spectrum of the energy donor overlaps with anabsorption spectrum of the energy acceptor (which relates to theintegral term).

Here, as already described with reference to FIGS. 1A to 1C, thephosphorescent compounds (the first and second phosphorescent compounds)are dispersed in the respective host materials and isolated from eachother by the host materials; thus, the distance R is at least onemolecule length or longer (i.e., 1 nm or more). Therefore, theexcitation energy generated in the first phosphorescent compound is notentirely transferred to the second phosphorescent compound by Förstermechanism. Meanwhile, Förster energy transfer to the distance R canoccur when R is less than or equal to approximately 10 nm to 20 nm. Sothat the distance R at least longer than or equal to one molecule lengthis secured between the first and second phosphorescent compounds, themass of the phosphorescent compound dispersed in the host material ispreferably set to less than or equal to a certain mass. Based on this,the concentration of the phosphorescent compound in the light-emittinglayer is 10 wt % or less. When the concentration of the phosphorescentcompound is too low, favorable characteristics are difficult to beachieved; thus, the concentration of the phosphorescent compound in thisembodiment is preferably greater than or equal to 0.1 wt % and less thanor equal to 10 wt %. Specifically, it is more preferable that the firstphosphorescent compound be contained in the first light-emitting layer113 a at a concentration of greater than or equal to 0.1 wt % and lessthan or equal to 5 wt %.

FIGS. 2A and 2B schematically illustrate Förster energy transfer betweenthe phosphorescent compounds in the light-emitting element of oneembodiment of the present invention, in which the first phosphorescentcompound 113Da and the second phosphorescent compound 113Db emittinglight with a wavelength longer than that of light emitted from the firstphosphorescent compound are included. In each of FIGS. 2A and 2B, astructure in which the first light-emitting layer 113 a and the secondlight-emitting layer 113 b are stacked between an electrode 10 and anelectrode 11 is illustrated. Note that one of the electrode 10 and theelectrode 11 functions as an anode and the other functions as a cathode.As illustrated in FIG. 2A, first, a singlet excited state formed in thefirst phosphorescent compound 113Da (S_(a)) is converted into a tripletexcited state (T_(a)) by intersystem crossing. In other words, anexciton in the first light-emitting layer 113 a is basically broughtinto T_(a).

Then, the energy of the exciton in the T_(a) state, some of which isconverted into light emission, can be partly transferred to the tripletexcited state of the second phosphorescent compound 113Db (T_(b)) byFörster mechanism. This results from the fact that the firstphosphorescent compound 113Da has a light-emitting property (has a highphosphorescence quantum yield φ) and that direct absorption, whichcorresponds to electron transition from a singlet ground state to atriplet excited state, is observed in the second phosphorescent compound113Db (an absorption spectrum of a triplet excited state exists). Whenthese conditions are fulfilled, triplet-triplet Förster energy transferfrom T_(a) to T_(b) is possible.

Note that a singlet excited state of the second phosphorescent compound113Db (S_(b)) has higher energy than the triplet excited state of thefirst phosphorescent compound 113Da (T_(a)) in many cases and thereforedoes not contribute to the above energy transfer so much in many cases.For this reason, the description is omitted here. It is needless to saythat when the singlet excited state of the second phosphorescentcompound 113Db (S_(b)) has lower energy than the triplet excited stateof the first phosphorescent compound 113Da (T_(a)), energy transfer mayoccur similarly. In that case, energy transferred to the singlet excitedstate of the second phosphorescent compound 113Db (S_(b)) is transferredto the triplet excited state of the second phosphorescent compound 113Db(T_(b)) by intersystem crossing to contribute to light emission.

Note that to make the above Förster energy transfer efficiently occurbetween the phosphorescent compounds serving as the dopants, not to thehost materials, it is preferable that absorption spectra of the firstand second host materials be not in the emission region of the firstphosphorescent compound 113Da. In this manner, when energy istransferred directly between dopants without being transferred throughthe host material (specifically, the second host material), formation ofan extra path of energy transfer is suppressed and high emissionefficiency can be achieved, which is preferable.

Further, the first host material preferably has a triplet excitationenergy higher than that of the first phosphorescent compound so as notto quench the first phosphorescent compound.

As described above, a basic concept of one embodiment of the presentinvention is an element structure in which the first phosphorescentcompound emitting light with the shorter wavelength is mainly excited ina state where the first and second phosphorescent compounds are isolatedfrom each other with the use of the host materials and the stacked-layerstructure. Since energy is partly transferred by Förster mechanism to acertain distance (20 nm or less) in such an element structure,excitation energy of the first phosphorescent compound is partlytransferred to the second phosphorescent compound. As a result, lightemission from each of the first and second phosphorescent compounds canbe obtained.

Here, what is more important in one embodiment of the present inventionis that the materials and the element structure are determined inconsideration of the above energy transfer.

To make Förster energy transfer occur, the energy donor needs to have ahigh luminescence quantum yield φ. In terms of the luminescence quantumyield, there is no problem in one embodiment of the present inventionsince a phosphorescent compound (specifically, a light-emitting compoundwith a phosphorescence quantum yield of 0.1 or more) is used. Animportant point is that the integral term of the formula (1) is madelarge, i.e., an emission spectrum F(ν) of the energy donor is made toproperly overlap with the molar absorption coefficient ε(ν) of theenergy acceptor.

In general, it is thought that the emission spectrum F(ν) of the energydonor simply needs to overlap with a wavelength range in which the molarabsorption coefficient ε(ν) of the energy acceptor is large (i.e., theproduct of F(ν) and ε(ν) simply needs to be large). However, this doesnot necessarily apply to Förster mechanism because the integral term inthe formula (1) is inversely proportional to the frequency ν raised tothe fourth power to have wavelength dependence.

For easier understanding, here, the formula (1) is transformed. Sinceλ=c/λ, where λ denotes a wavelength of light, the formula (1) can betransformed into a formula (2).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack & \; \\{\mspace{194mu}{k_{F} = {\frac{9000\mspace{11mu} K^{2}{\phi ln}\mspace{11mu} 10}{128\pi^{5}n^{4}N\;\tau\; R^{6}}{\int{{F(\lambda)}{ɛ(\lambda)}\lambda^{4}{\mathbb{d}\lambda}}}}}} & (2)\end{matrix}$

In other words, it can be found that the longer the wavelength λ, is,the larger the integral term is. In simpler terms, it is indicated thatenergy transfer occurs more easily on a longer wavelength side. That is,this is not so simple that F(λ) needs to overlap with the wavelengthrange in which the molar absorption coefficient ε(λ) is large. It isnecessary that F(λ) overlap with a range in which ε(λ)λ⁴ is large.

Thus, in the light-emitting element of one embodiment of the presentinvention, in order to increase efficiency of energy transfer from thefirst phosphorescent compound 113Da, a phosphorescent compound allowinga band having the maximum value of an emission spectrum of the firstphosphorescent compound 113Da to overlap with a band having thelongest-wavelength-side peak of the function ε(λ)λ⁴ of the secondphosphorescent compound 113Db is used as the second phosphorescentcompound 113Db.

Note that the wavelength of the longest-wavelength-side peak of thefunction ε(λ)λ⁴ of the second phosphorescent compound preferablyoverlaps with a phosphorescence spectrum F(λ) of the firstphosphorescent compound. Further, it is more preferable that awavelength range in which the band with the above maximum value of theemission spectrum of the first phosphorescent compound 113Da has half ofthe intensity of the above maximum value overlap with a wavelength rangein which the band having the above peak of the function ε(λ)λ⁴ of thesecond phosphorescent compound has half of the intensity of the abovepeak, in which case the overlap between the spectra can be larger.

In a light-emitting element having the above-described structure, highemission efficiency can be achieved and the phosphorescent compounds canprovide light emissions in a good balance.

For better understanding of such structures of phosphorescent compounds,explanation is made below referring to specific examples. Here, as anexample, a case is described where a compound (1) shown below(bis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: Ir(tBuppm)₂(acac))) is used as the first phosphorescentcompound 113Da and a compound (2) shown below(bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: Ir(tppr)₂(dpm))) is used as the second phosphorescentcompound 113Db which emits light with a wavelength longer than that oflight emitted from the first phosphorescent compound 113Da.

FIG. 3A shows a molar absorption coefficient ε(λ) and ε(λ)λ⁴ of thecompound (2) that is the second phosphorescent compound. The molarabsorption coefficient ε(λ) gets smaller on a longer wavelength side,but ε(λ)λ⁴ has the peak at around 550 nm (which corresponds to thetriplet MLCT absorption band of the compound (2)). As can be seen fromthis example, affected by the term λ⁴, ε(λ)λ⁴ of the secondphosphorescent compound has the peak in the absorption band (tripletMLCT absorption band) located on the longest wavelength side.

FIG. 3B shows a photoluminescence (PL) spectrum F(λ) of the compound (1)and ε(λ)λ⁴ of the compound (2). The compound (1) is the firstphosphorescent compound and emits green light with an emission peak ataround 545 nm. Around the longest-wavelength-side peak of ε(λ)λ⁴ of thesecond phosphorescent compound, the PL spectrum F(λ) of the firstphosphorescent compound largely overlaps with ε(λ)λ⁴, and energytransfer from the first phosphorescent compound to the secondphosphorescent compound occurs by Förster mechanism. Note that in thiscase, since the peak corresponds to the triplet MLCT absorption band,the energy transfer is the triplet-triplet Förster energy transfer(T_(a)-T_(b) energy transfer in FIGS. 2A and 2B). At this time, thedifference between the emission peak wavelength of the PL spectrum F(λ)of the first phosphorescent compound and the wavelength of thelongest-wavelength-side peak of ε(λ)λ⁴ of the second phosphorescentcompound is preferably 0.2 eV or less, in which case energy transferoccurs efficiently. The emission peak wavelength of the PL spectrum F(λ)of the compound (1) is 546 nm and the wavelength of thelongest-wavelength-side peak of ε(λ)λ⁴ of the compound (2) is 543 nm,with the difference of 3 nm, which corresponds to 0.01 eV. Thus, itturns out energy transfer between the compound (1) and the compound (2)occurs very efficiently.

Note that from the above, it is preferable that absorption spectrum ofthe second phosphorescent compound shows, on the longest wavelengthside, direct absorption which corresponds to electron transition from asinglet ground state to a triplet excited state (e.g., triplet MLCTabsorption). Such a structure leads to high efficiency oftriplet-triplet energy transfer shown in FIGS. 2A and 2B.

To obtain the above-described recombination region, in the case wherethe first light-emitting layer 113 a is positioned on the anode side, atleast the second light-emitting layer 113 b preferably has anelectron-transport property, and both the first light-emitting layer 113a and the second light-emitting layer 113 b may have anelectron-transport property. In the case where the first light-emittinglayer 113 a is positioned on the cathode side, at least the secondlight-emitting layer 113 b preferably has a hole-transport property, andboth the first light-emitting layer 113 a and the second light-emittinglayer 113 b may have a hole-transport property.

In addition, in the first light-emitting layer 113 a, it is preferablethat a band having a peak of the photoluminescence (PL) spectrum F(λ) ofthe first host material 113Ha largely overlap with a band having thelongest-wavelength-side peak of the function ε(λ)λ⁴ of the firstphosphorescent compound.

However, in general, it is difficult to obtain an overlap between a bandhaving a peak of a photoluminescence (PL) spectrum F(λ) of a hostmaterial and a band having the longest-wavelength-side peak of thefunction ε(λ)λ⁴ of a guest material (the first phosphorescent compound113Da). The reason for this is that since photoluminescence (PL) of thehost material is generally fluorescence, which is light emission from anenergy level higher than that of phosphorescence, there is highpossibility that a triplet excitation energy level of the host materialwhose fluorescence spectrum is at a wavelength in close proximity to anabsorption spectrum on the longest wavelength side of the guest material(a triplet excited state of a guest material) becomes lower than atriplet excitation energy level of the guest material. When the tripletexcitation energy level of the host material becomes lower than thetriplet excitation energy level of the guest material, the tripletexcitation energy of the guest material is transferred to the hostmaterial, which causes a reduction in emission efficiency.

Thus, in this embodiment, it is preferable that the first light-emittinglayer 113 a further contain the first organic compound 113A and thefirst host material 113Ha and the first organic compound 113A form anexciplex (also referred to as excited complex) 113Ec (FIG. 1B and FIG.2B). In FIG. 2B, 10 and 11 denote electrodes, one of which functions asan anode and the other of which functions as a cathode. Note that in thedrawings, a singlet excited state and a triplet excited state of theexciplex 113Ec are represented by S_(e) and T_(e), respectively, asinglet excited state and a triplet excited state of the firstphosphorescent compound 113Da are represented by S_(a) and T_(a),respectively, and a singlet excited state and a triplet excited state ofthe second phosphorescent compound 113Db are represented by S_(b) andT_(b), respectively.

In that case, at the time of recombination of carriers (electron andhole) in the first light-emitting layer 113 a, the first organiccompound 113A and the first host material 113Ha form the exciplex 113Ecby receiving energy as a result of recombination of electrons and holes.Fluorescence from the exciplex 113Ec has a spectrum on a longerwavelength side with respect to a fluorescence spectrum of the firstorganic compound 113A alone and a fluorescence spectrum of the firsthost material 113Ha alone, and also has a characteristic in that thesinglet excited state S_(e) and the triplet excited state T_(e) of theexciplex 113Ec are extremely close to each other in terms of energy.Therefore, when a band having a peak of a PL spectrum F(λ) which showslight emission from the singlet excited state of the exciplex 113Ecoverlaps with the band having the longest-wavelength-side peak of thefunction ε(λ)λ⁴ of the guest material (the first phosphorescent compound113Da) (which corresponds to an absorption spectrum of a triplet excitedstate T_(a) of the guest material), both energy transfer from S_(e) toT_(a) and energy transfer from T_(e) to T_(a) can be enhanced as much aspossible. In this case, a difference between an emission peak wavelengthof the exciplex 113Ec and a wavelength of a peak of ε(λ)λ⁴ of the guestmaterial (the first phosphorescent compound 113Da) is preferably 0.2 eVor less, in which case energy transfer occurs efficiently. Moreover, thetriplet excitation energy levels of the first organic compound 113A andthe first host material 113Ha are preferably kept higher than thetriplet excitation energy level of the first phosphorescent compound113Da.

Part of the energy thus transferred to the first phosphorescent compound113Da is transferred to the second phosphorescent compound 113Db asdescribed above, so that both the first phosphorescent compound 113Daand the second phosphorescent compound 113Db emit light efficiently.

Note that energy transfer from the triplet excited state (T_(e)) of theexciplex 113Ec to the first phosphorescent compound 113Da efficientlyoccurs by Dexter mechanism Energy transfer from the singlet excitedstate (S_(e)) efficiently occurs by the above-described Förstermechanism, whereby efficient energy transfer is achieved as a whole.

There is no particular limitation on the first organic compound 113A andthe first host material 113Ha as long as they can form an exciplex; acombination of a compound which is likely to accept electrons (acompound having an electron-trapping property) and a compound which islikely to accept holes (a compound having a hole-trapping property) ispreferably employed.

The following are examples of the compound which is likely to acceptelectrons: a metal complex such asbis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); aheterocyclic compound having a polyazole skeleton such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), or2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); a heterocyclic compound having a diazineskeleton such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine(abbreviation: 4,6mPnP2Pm), or4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation:4,6mDBTP2Pm-II); and a heterocyclic compound having a pyridine skeletonsuch as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation:35DCzPPy) or 1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation:TmPyPB). Among the above materials, a heterocyclic compound having adiazine skeleton and a heterocyclic compound having a pyridine skeletonhave high reliability and are thus preferable. Specifically, aheterocyclic compound having a diazine (pyrimidine or pyrazine) skeletonhas a high electron-transport property to contribute to a reduction indrive voltage.

The following are examples of the compound which is likely to acceptholes: a compound having an aromatic amine skeleton such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1′,1′-biphenyl]-4,4′-diamine(abbreviation: TPD),4,4′-bis[N-(Spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBi1BP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine(abbreviation: PCBAF), orN-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF); a compound having a carbazole skeleton such as1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), or3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); a compound havinga thiophene skeleton such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III), or4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV); and a compound having a furan skeleton suchas 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation:DBF3P-II) or4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II). Among the above materials, a compoundhaving an aromatic amine skeleton and a compound having a carbazoleskeleton are preferable because these compounds are highly reliable andhave high hole-transport properties to contribute to a reduction indrive voltage.

The first organic compound 113A and the first host material 113Ha arenot limited to those compounds as long as they can form an exciplex, theband having the peak of the photoluminescence (PL) spectrum F(λ) of theexciplex overlaps with the band having the longest-wavelength-side peakof the function ε(λ)λ⁴ of the first phosphorescent compound, and thepeak of an emission spectrum of the exciplex has a longer wavelengththan a peak of the emission spectrum of the first phosphorescentcompound 113Da.

Note that in the case where a compound which is likely to acceptelectrons and a compound which is likely to accept holes are used forthe first organic compound 113A and the first host material 113Ha,carrier balance can be controlled by the mixture ratio of the compounds.Specifically, the ratio of the first organic compound 113A to the firsthost material 113Ha is preferably from 1:9 to 9:1.

Here, in this structure, the first host material 113Ha and the firstorganic compound 113A are selected such that an exciplex is formed whichallows the band having the peak of the photoluminescence (PL) spectrumF(λ) of the exciplex to overlap with the band having thelongest-wavelength-side peak of the function ε(λ)λ⁴ of the firstphosphorescent compound. It is preferable that the overlap between thebands be as large as possible.

Note that a wavelength of the longest-wavelength-side peak of thefunction ε(λ)λ⁴ of the first phosphorescent compound preferably overlapswith the photoluminescence (PL) spectrum F(λ) of the exciplex. Further,it is more preferable that a wavelength range in which the band with thepeak of the photoluminescence (PL) spectrum F(λ) of the exciplex hashalf of the intensity of the above peak overlap with a wavelength rangein which the band with the peak of the function ε(λ)λ⁴ of the firstphosphorescent compound has half of the intensity of the above peak, inwhich case the overlap between the spectra can be larger.

In the structure, energy can be efficiently transferred from theexciplex formed from the first host material 113Ha and the first organiccompound 113A to the first phosphorescent compound 113Da to enhance theenergy transfer efficiency, whereby a light-emitting element with ahigher external quantum efficiency can be obtained.

Embodiment 2

In this embodiment, a detailed example of the structure of thelight-emitting element described in Embodiment 1 will be described belowwith reference to FIGS. 1A to 1C.

A light-emitting element in this embodiment includes, between a pair ofelectrodes, an EL layer including a plurality of layers. In thisembodiment, the light-emitting element includes the first electrode 101,the second electrode 102, and the EL layer 103, which is providedbetween the first electrode 101 and the second electrode 102. Note thatin this embodiment, description is made on the assumption that the firstelectrode 101 functions as an anode and that the second electrode 102functions as a cathode. In other words, when a voltage is appliedbetween the first electrode 101 and the second electrode 102 so that thepotential of the first electrode 101 is higher than that of the secondelectrode 102, light emission can be obtained.

Since the first electrode 101 functions as the anode, the firstelectrode 101 is preferably formed using any of metals, alloys,electrically conductive compounds with a high work function(specifically, a work function of 4.0 eV or more), mixtures thereof, andthe like. Specifically, for example, indium oxide-tin oxide (ITO: indiumtin oxide), indium oxide-tin oxide containing silicon or silicon oxide,indium oxide-zinc oxide, indium oxide containing tungsten oxide and zincoxide (IWZO), and the like can be given. Films of these electricallyconductive metal oxides are usually formed by a sputtering method butmay be formed by application of a sol-gel method or the like. In anexample of the formation method, indium oxide-zinc oxide is deposited bya sputtering method using a target obtained by adding 1 wt % to 20 wt %of zinc oxide to indium oxide. Further, a film of indium oxidecontaining tungsten oxide and zinc oxide (IWZO) can be formed by asputtering method using a target in which tungsten oxide and zinc oxideare added to indium oxide at 0.5 wt % to 5 wt % and 0.1 wt % to 1 wt %,respectively. Besides, gold (Au), platinum (Pt), nickel (Ni), tungsten(W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper(Cu), palladium (Pd), nitrides of metal materials (e.g., titaniumnitride), and the like can be given. Graphene can also be used. Notethat when a composite material described later is used for a layer whichis in contact with the first electrode 101 in the EL layer 103, anelectrode material can be selected regardless of its work function.

There is no particular limitation on the stacked-layer structure of theEL layer 103 as long as the EL layer includes the light-emitting layer113 which has a structure similar to that described in Embodiment 1. Forexample, the EL layer 103 can be formed by combining a hole-injectionlayer, a hole-transport layer, the light-emitting layer, anelectron-transport layer, an electron-injection layer, acarrier-blocking layer, an intermediate layer, and the like asappropriate. In this embodiment, the EL layer 103 has a structure inwhich the hole-injection layer 111, the hole-transport layer 112, thelight-emitting layer 113, the electron-transport layer 114, and theelectron-injection layer 115 are stacked in this order over the firstelectrode 101. Materials included in the layers are specifically givenbelow.

The hole-injection layer 111 is a layer containing a substance having ahigh hole-injection property. Molybdenum oxide, vanadium oxide,ruthenium oxide, tungsten oxide, manganese oxide, or the like can beused. Alternatively, the hole-injection layer 111 can be formed using aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc) or copper phthalocyanine (abbreviation: CuPc), an aromatic aminecompound such as4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB) orN,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD), a high molecular compound such aspoly(ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS), orthe like.

Alternatively, a composite material in which a substance having ahole-transport property contains a substance having an acceptor propertycan be used for the hole-injection layer 111. Note that the use of sucha substance having a hole-transport property which contains a substancehaving an acceptor property enables selection of a material used to forman electrode regardless of its work function. In other words, besides amaterial having a high work function, a material having a low workfunction can also be used for the first electrode 101. As the substancehaving an acceptor property,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. In addition, transitionmetal oxides can be given. Oxides of the metals that belong to Group 4to Group 8 of the periodic table can be given. Specifically, vanadiumoxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, and rhenium oxide are preferable inthat their electron-accepting property is high. Among these, molybdenumoxide is particularly preferable in that it is stable in the air, has alow hygroscopic property, and is easily treated.

As the substance having a hole-transport property used for the compositematerial, any of a variety of organic compounds such as aromatic aminecompounds, carbazole derivatives, aromatic hydrocarbons, and highmolecular compounds (e.g., oligomers, dendrimers, or polymers) can beused. Note that the organic compound used for the composite material ispreferably an organic compound having a high hole-transport property.Specifically, a substance having a hole mobility of 10⁻⁶ cm²/Vs or moreis preferably used. Organic compounds that can be used as the substancehaving a hole-transport property in the composite material arespecifically given below.

Examples of the aromatic amine compounds areN,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB),N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), and the like.

Specific examples of the carbazole derivatives that can be used for thecomposite material are3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like.

Other examples of the carbazole derivatives that can be used for thecomposite material are 4,4′-di(N-carbazolyl)biphenyl (abbreviation:CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and thelike.

Examples of the aromatic hydrocarbons that can be used for the compositematerial are 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation:t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, andthe like. Besides, pentacene, coronene, or the like can also be used.The aromatic hydrocarbon which has a hole mobility higher than or equalto 1×10⁻⁶ cm²/Vs and which has 14 to 42 carbon atoms is particularlypreferable.

Note that the aromatic hydrocarbons that can be used for the compositematerial may have a vinyl skeleton. Examples of the aromatic hydrocarbonhaving a vinyl group are 4,4′-bis(2,2-diphenylvinyl)biphenyl(abbreviation: DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene(abbreviation: DPVPA), and the like.

A high molecular compound such as poly(N-vinylcarbazole) (abbreviation:PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:poly-TPD) can also be used.

By providing a hole-injection layer, a high hole-transport property canbe achieved to allow a light-emitting element to have a small drivevoltage.

The hole-transport layer 112 is a layer that contains a substance havinga hole-transport property. Examples of the substance having ahole-transport property are aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N′-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), and the like. The substances mentioned here havehigh hole-transport properties and are mainly ones that have a holemobility of 10⁻⁶ cm²/Vs or more. An organic compound given as an exampleof the substance having a hole-transport property in the compositematerial described above can also be used for the hole-transport layer112. A high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK) or poly(4-vinyltriphenylamine) (abbreviation: PVTPA)can also be used. Note that the layer that contains a substance having ahole-transport property is not limited to a single layer, and may be astack of two or more layers including any of the above substances.

The light-emitting layer 113 is a layer containing the firstphosphorescent compound and the second phosphorescent compound. Sincethe light-emitting layer 113 has a structure similar to that describedin Embodiment 1, the light-emitting element in this embodiment can havean extremely high emission efficiency. Embodiment 1 is to be referred tofor main structures of the light-emitting layer 113.

There is no particular limitation on materials that can be used as thefirst phosphorescent compound and the second phosphorescent compound inthe light-emitting layer 113 as long as they have the relation describedin Embodiment 1. The following can be given as examples of the firstphosphorescent compound and the second phosphorescent compound.

The examples are an organometallic iridium complex having a 4H-triazoleskeleton such as tris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN²]phenyl-κC}iridium(III)(abbreviation: Ir(mpptz-dmp)₃),tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Mptz)₃), ortris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(iPrptz-3b)₃); an organometallic iridium complex havinga 1H-triazole skeleton such astris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(Mptz1-mp)₃), ortris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Prptz1-Me)₃); an organometallic iridium complex havingan imidazole skeleton such asfac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III)(abbreviation: Ir(iPrpmi)₃) ortris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III)(abbreviation: Ir(dmpimpt-Me)₃); and an organometallic iridium complexin which a phenylpyridine derivative having an electron-withdrawinggroup is a ligand, such asbis[2-(4′,6′-difiuorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) picolinate(abbreviation: FIrpic),bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(III)picolinate (abbreviation: Ir(CF₃ppy)₂(pic)), orbis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIracac). These are compounds emittingblue phosphorescence and have an emission peak at 440 nm to 520 nm.Among the above compounds, an organometallic iridium complex having apolyazole skeleton such as a 4H-triazole skeleton, a 1H-triazoleskeleton, or an imidazole skeleton has a high hole-trapping property.Therefore, it is preferable that any of these compounds be used as thefirst phosphorescent compound in the light-emitting element of oneembodiment of the present invention, the first light-emitting layer beprovided closer to the cathode than the second light-emitting layer, andthe second light-emitting layer have a hole-transport property(specifically, the second host material be a hole-transport material),in which case a recombination region of carriers can be easilycontrolled to be in the first light-emitting layer. Note that anorganometallic iridium complex having a 4H-triazole skeleton hasexcellent reliability and emission efficiency and thus is particularlypreferable.

Other examples are an organometallic iridium complex having a pyrimidineskeleton such as tris(4-methyl-6-phenylpyrimidinato)iridium(III)(abbreviation: Ir(mppm)₃),tris(4-t-butyl-6-phenylpyrimidinato)iridium(III) (abbreviation:Ir(tBuppm)₃),(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(mppm)₂(acac)),(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)),(acetylacetonato)bis[6-(2-norbornyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: Ir(nbppm)₂(acac)),(acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: Ir(mpmppm)₂(acac)), or(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: Ir(dppm)₂(acac)); an organometallic iridium complexhaving a pyrazine skeleton such as(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-Me)₂(acac)) or(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-iPr)₂(acac)); an organometallic iridium complexhaving a pyridine skeleton such astris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃),bis(2-phenylpyridinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(ppy)₂(acac)), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: Ir(bzq)₂(acac)),tris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq)₃),tris(2-phenylquinolinato-N,C^(2′))iridium(III) (abbreviation: Ir(pq)₃),or bis(2-phenylquinolinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(pq)₂(acac)); and a rare earth metal complex such astris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation:Tb(acac)₃(Phen)). These are mainly compounds emitting greenphosphorescence and have an emission peak at 500 nm to 600 nm. Among theabove compounds, an organometallic iridium complex having a diazineskeleton such as a pyrimidine skeleton or a pyrazine skeleton has a lowhole-trapping property and a high electron-trapping property. Therefore,it is preferable that any of these compounds be used as the firstphosphorescent compound in the light-emitting element of one embodimentof the present invention, the first light-emitting layer be providedcloser to the anode than the second light-emitting layer, and the secondlight-emitting layer have an electron-transport property (specifically,the second host material be an electron-transport material), in whichcase a recombination region of carriers can be easily controlled to bein the first light-emitting layer. Note that an organometallic iridiumcomplex having a pyrimidine skeleton has distinctively high reliabilityand emission efficiency and thus is particularly preferable.

Still other examples are an organometallic iridium complex having apyrimidine skeleton such asbis[4,6-bis(3-methylphenyl)pyrimidinato](diisobutylylmethano)iridium(III)(abbreviation: Ir(5mdppm)₂(dibm)),bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: Ir(5mdppm)₂(dpm)), orbis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: Ir(d1npm)₂(dpm)); an organometallic iridium complexhaving a pyrazine skeleton such as(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)),bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: Ir(tppr)₂(dpm)), or(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)); an organometallic iridium complexhaving a pyridine skeleton such astris(1-phenylisoquinolinato-N,C^(2′))iridium(III) (abbreviation:Ir(piq)₃) or bis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate (abbreviation: Ir(piq)₂(acac)); a platinum complex suchas 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP); and a rare earth metal complex such astris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)) ortris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)). These are compounds emitting redphosphorescence and have an emission peak at 600 nm to 700 nm. Among theabove materials, an organometallic iridium complex having a diazineskeleton such as a pyrimidine skeleton or a pyrazine skeleton has a lowhole-trapping property and a high electron-trapping property. Therefore,it is preferable that an organometallic iridium complex having a diazineskeleton be used as the second phosphorescent compound, the firstlight-emitting layer be provided closer to the cathode than the secondlight-emitting layer, and the second light-emitting layer have ahole-transport property (specifically, the second host material be ahole-transport material), in which case a recombination region ofcarriers can be easily controlled to be in the first light-emittinglayer. Note that an organometallic iridium complex having a pyrimidineskeleton has distinctively high reliability and emission efficiency andthus is particularly preferable. Further, because an organometalliciridium complex having a pyrazine skeleton can provide red lightemission with favorable chromaticity, the use of the organometalliciridium complex in a white light-emitting element improves a colorrendering property of the white light-emitting element.

It is also possible to select a first phosphorescent material and asecond phosphorescent material which have the relation described inEmbodiment 1, from known phosphorescent materials in addition to theabove phosphorescent compounds.

Note that instead of phosphorescent compounds (the first phosphorescentcompound 113 a and the second phosphorescent compound 113 b), materialsexhibiting thermally activated delayed fluorescence, i.e., thermallyactivated delayed fluorescent (TADF) materials, may be used. Here, theterm “delayed fluorescence” refers to light emission having a spectrumsimilar to that of normal fluorescence and an extremely long lifetime.The lifetime is 10⁻⁶ seconds or longer, preferably 10⁻³ seconds orlonger. Specific examples of the thermally activated delayed fluorescentmaterials include a fullerene, a derivative thereof, an acridinederivative such as proflavine, and eosin. Besides, a metal-containingporphyrin can be used, such as a porphyrin containing magnesium (Mg),zinc (Zn), cadmium (Cd), tin (Sn), platinum (Pt), indium (In), orpalladium (Pd). Examples of the metal-containing porphyrin include aprotoporphyrin-tin fluoride complex (abbreviation: SnF₂(Proto IX)), amesoporphyrin-tin fluoride complex (abbreviation: SnF₂(Meso IX)), ahematoporphyrin-tin fluoride complex (abbreviation: SnF₂(Hemato IX)), acoproporphyrin tetramethyl ester-tin fluoride complex (abbreviation:SnF₂(Copro III-4Me)), an octaethylporphyrin-tin fluoride complex(abbreviation: SnF₂(OEP)), an etioporphyrin-tin fluoride complex(abbreviation: SnF₂(Etio I)), and an octaethylporphyrin-platinumchloride complex (abbreviation: PtCl₂(OEP)). Alternatively, aheterocyclic compound including a π-electron rich heteroaromatic ringand a π-electron deficient heteroaromatic ring can be used, such as2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-α]carbazol-11-yl)-1,3,5-triazine(abbreviation: PIC-TRZ). Note that a material in which the π-electronrich heteroaromatic ring is directly bonded to the π-electron deficientheteroaromatic ring is particularly preferably used because the donorproperty of the π-electron rich heteroaromatic ring and the acceptorproperty of the π-electron deficient heteroaromatic ring are bothincreased and the energy difference between the S₁ level and the T₁level becomes small.

There is no particular limitation on the materials which can be used asthe first and second host materials; a variety of carrier transportingmaterials may be selected and appropriately combined such that theelement structure illustrated in FIGS. 1A to 1C is obtained.

The following are examples of the host material having anelectron-transport property: a metal complex such asbis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); aheterocyclic compound having a polyazole skeleton such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), or2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBIBIm-II); a heterocyclic compound having a diazineskeleton such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine(abbreviation: 4,6mPnP2Pm), or4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation:4,6mDBTP2Pm-II); and a heterocyclic compound having a pyridine skeletonsuch as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation:35DCzPPy) or 1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation:TmPyPB). Among the above materials, a heterocyclic compound having adiazine skeleton and a heterocyclic compound having a pyridine skeletonhave high reliability and are thus preferable. Specifically, aheterocyclic compound having a diazine (pyrimidine or pyrazine) skeletonhas a high electron-transport property to contribute to a reduction indrive voltage.

The following are examples of the host material having a hole-transportproperty: a compound having an aromatic amine skeleton such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBi1BP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine(abbreviation: PCBAF), orN-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF); a compound having a carbazole skeleton such as1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), or3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); a compound havinga thiophene skeleton such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III), or4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV); and a compound having a furan skeleton suchas 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation:DBF3P-II) or4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II). Among the above materials, a compoundhaving an aromatic amine skeleton and a compound having a carbazoleskeleton are preferable because these compounds are highly reliable andhave high hole-transport properties to contribute to a reduction indrive voltage.

Host materials can be selected from known substances as well as from theabove host materials. Note that as the host materials, substances havinga triplet level (energy gap between a ground state and a triplet excitedstate) higher than that of the phosphorescent compound are preferablyselected. It is preferable that these host materials do not have anabsorption spectrum in the blue wavelength range. Specifically, anabsorption edge of the absorption spectrum is preferably at 440 nm orless.

For formation of the light-emitting layer 113 having the above-describedstructure, co-evaporation by a vacuum evaporation method can be used, oralternatively an inkjet method, a spin coating method, a dip coatingmethod, or the like using a mixed solution can be used.

The electron-transport layer 114 is a layer containing a substancehaving an electron-transport property. For example, a layer containing ametal complex having a quinoline skeleton or a benzoquinoline skeleton,such as tris(8-quinolinolato)aluminum (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq), or the like can be used. Alternatively, a metal complex having anoxazole-based or thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)₂), orthe like can be used. Besides the metal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can also be used. Thesubstances mentioned here have high electron-transport properties andare mainly ones that have an electron mobility of 10⁻⁶ cm²/Vs or more.Note that any of the above-described host materials havingelectron-transport properties may be used for the electron-transportlayer 114.

Furthermore, the electron-transport layer 114 is not limited to a singlelayer and may be a stack of two or more layers containing any of theabove substances.

Between the electron-transport layer and the light-emitting layer, alayer that controls transport of electron carriers may be provided. Thisis a layer formed by addition of a small amount of a substance having ahigh electron-trapping property to a material having a highelectron-transport property as described above, and the layer is capableof adjusting carrier balance by suppressing transport of electroncarriers. Such a structure is very effective in preventing a problem(such as a reduction in element lifetime) caused when electrons passthrough the light-emitting layer.

In addition, the electron-injection layer 115 may be provided in contactwith the second electrode 102 between the electron-transport layer 114and the second electrode 102. For the electron-injection layer 115, analkali metal, an alkaline earth metal, or a compound thereof such aslithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride(CaF₂) can be used. For example, a layer that is formed using asubstance having an electron-transport property and contains an alkalimetal, an alkaline earth metal, or a compound thereof can be used. Notethat a layer that is formed using a substance having anelectron-transport property and contains an alkali metal or an alkalineearth metal is preferably used as the electron-injection layer 115, inwhich case electron injection from the second electrode 102 isefficiently performed.

For the second electrode 102, any of metals, alloys, electricallyconductive compounds, and mixtures thereof which have a low workfunction (specifically, a work function of 3.8 eV or less) or the likecan be used. Specific examples of such a cathode material are elementsbelonging to Groups 1 and 2 of the periodic table, such as alkali metals(e.g., lithium (Li) and cesium (Cs)), magnesium (Mg), calcium (Ca), andstrontium (Sr), alloys thereof (e.g., MgAg and AlLi), rare earth metalssuch as europium (Eu) and ytterbium (Yb), alloys thereof, and the like.However, when the electron-injection layer is provided between thesecond electrode 102 and the electron-transport layer, for the secondelectrode 102, any of a variety of conductive materials such as Al, Ag,ITO, or indium oxide-tin oxide containing silicon or silicon oxide canbe used regardless of the work function. Films of these electricallyconductive materials can be formed by a sputtering method, an inkjetmethod, a spin coating method, or the like.

Further, any of a variety of methods can be used to form the EL layer103 regardless whether it is a dry process or a wet process. Forexample, a vacuum evaporation method, an inkjet method, a spin coatingmethod, or the like may be used. Different formation methods may be usedfor the electrodes or the layers.

In addition, the electrode may be formed by a wet method using a sol-gelmethod, or by a wet method using paste of a metal material.Alternatively, the electrode may be formed by a dry method such as asputtering method or a vacuum evaporation method.

In the light-emitting element having the above-described structure,current flows due to a potential difference between the first electrode101 and the second electrode 102, and holes and electrons recombine inthe light-emitting layer 113 which contains a substance having a highlight-emitting property, so that light is emitted. That is, alight-emitting region is formed in the light-emitting layer 113.

Light emission is extracted out through one or both of the firstelectrode 101 and the second electrode 102. Therefore, one or both ofthe first electrode 101 and the second electrode 102 arelight-transmitting electrodes. In the case where only the firstelectrode 101 is a light-transmitting electrode, light emission isextracted through the first electrode 101. In the case where only thesecond electrode 102 is a light-transmitting electrode, light emissionis extracted through the second electrode 102. In the case where boththe first electrode 101 and the second electrode 102 arelight-transmitting electrodes, light emission is extracted through thefirst electrode 101 and the second electrode 102.

The structure of the layers provided between the first electrode 101 andthe second electrode 102 is not limited to the above-describedstructure. Preferably, a light-emitting region where holes and electronsrecombine is positioned away from the first electrode 101 and the secondelectrode 102 so that quenching due to the proximity of thelight-emitting region and a metal used for electrodes andcarrier-injection layers can be prevented.

Further, in order that transfer of energy from an exciton generated inthe light-emitting layer can be suppressed, preferably, thehole-transport layer and the electron-transport layer which are incontact with the light-emitting layer 113, particularly acarrier-transport layer in contact with a side closer to thelight-emitting region in the light-emitting layer 113 is formed using asubstance having a wider band gap than the light-emitting substance ofthe light-emitting layer or the emission center substance included inthe light-emitting layer.

A light-emitting element in this embodiment is preferably fabricatedover a substrate of glass, plastic, or the like. As the way of stackinglayers over the substrate, layers may be sequentially stacked from thefirst electrode 101 side or sequentially stacked from the secondelectrode 102 side. In a light-emitting device, although onelight-emitting element may be formed over one substrate, a plurality oflight-emitting elements may be formed over one substrate. With aplurality of light-emitting elements as described above formed over onesubstrate, a lighting device in which elements are separated or apassive-matrix light-emitting device can be manufactured. Alight-emitting element may be formed over an electrode electricallyconnected to a thin film transistor (TFT), for example, which is formedover a substrate of glass, plastic, or the like, so that an activematrix light-emitting device in which the TFT controls the drive of thelight-emitting element can be manufactured. Note that there is noparticular limitation on the structure of the TFT, which may be astaggered TFT or an inverted staggered TFT. In addition, crystallinityof a semiconductor used for the TFT is not particularly limited either;an amorphous semiconductor or a crystalline semiconductor may be used.In addition, a driver circuit formed in a TFT substrate may be formedwith an n-type TFT and a p-type TFT, or with either an n-type TFT or ap-type TFT.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 3

In this embodiment, a light-emitting device using the light-emittingelement described in Embodiments 1 and 2 will be described.

In this embodiment, the light-emitting device using the light-emittingelement described in Embodiments 1 and 2 is described with reference toFIGS. 4A and 4B. Note that FIG. 4A is a top view of the light-emittingdevice and FIG. 4B is a cross-sectional view taken along the lines A-Band C-D in FIG. 4A. This light-emitting device includes a driver circuitportion (source line driver circuit) 601, a pixel portion 602, and adriver circuit portion (gate line driver circuit) 603, which are tocontrol light emission of the light-emitting element and illustratedwith dotted lines. Moreover, a reference numeral 604 denotes a sealingsubstrate; 625, a drying agent; 605, a sealing material; and 607, aspace surrounded by the sealing material 605.

Reference numeral 608 denotes a wiring for transmitting signals to beinputted into the source line driver circuit 601 and the gate linedriver circuit 603 and receiving signals such as a video signal, a clocksignal, a start signal, and a reset signal from an FPC (flexible printedcircuit) 609 serving as an external input terminal. Although only theFPC is illustrated here, a printed wiring board (PWB) may be attached tothe FPC. The light-emitting device in the present specificationincludes, in its category, not only the light-emitting device itself butalso the light-emitting device provided with the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG.4B. The driver circuit portion and the pixel portion are formed over anelement substrate 610; the source line driver circuit 601, which is adriver circuit portion, and one of the pixels in the pixel portion 602are illustrated here.

As the source line driver circuit 601, a CMOS circuit in which ann-channel TFT 623 and a p-channel TFT 624 are combined is formed. Inaddition, the driver circuit may be formed with any of a variety ofcircuits such as a CMOS circuit, a PMOS circuit, or an NMOS circuit.Although a driver integrated type in which the driver circuit is formedover the substrate is illustrated in this embodiment, the driver circuitis not necessarily formed over the substrate, and the driver circuit canbe formed outside, not over the substrate.

The pixel portion 602 includes a plurality of pixels including aswitching TFT 611, a current controlling TFT 612, and a first electrode613 electrically connected to a drain of the current controlling TFT612. Note that to cover an end portion of the first electrode 613, aninsulator 614 is formed, for which a positive photosensitive acrylicresin film is used here.

In order to improve coverage, the insulator 614 is formed to have acurved surface with curvature at its upper or lower end portion. Forexample, in the case where positive photosensitive acrylic is used for amaterial of the insulator 614, only the upper end portion of theinsulator 614 preferably has a curved surface with a curvature radius(0.2 μm to 3 μm). As the insulator 614, either a negative photosensitiveresin or a positive photosensitive resin can be used.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. Here, as a material used for the first electrode 613functioning as an anode, a material having a high work function ispreferably used. For example, a single-layer film of an ITO film, anindium tin oxide film containing silicon, an indium oxide filmcontaining zinc oxide at 2 wt % to 20 wt %, a titanium nitride film, achromium film, a tungsten film, a Zn film, a Pt film, or the like, astack of a titanium nitride film and a film containing aluminum as itsmain component, a stack of three layers of a titanium nitride film, afilm containing aluminum as its main component, and a titanium nitridefilm, or the like can be used. The stacked-layer structure enables lowwiring resistance, favorable ohmic contact, and a function as an anode.

In addition, the EL layer 616 is formed by any of a variety of methodssuch as an evaporation method using an evaporation mask, an inkjetmethod, and a spin coating method. The EL layer 616 has a structuresimilar to that described in Embodiments 1 and 2. Further, for anothermaterial included in the EL layer 616, any of low molecular compoundsand high molecular compounds (including oligomers and dendrimers) may beused.

As a material used for the second electrode 617, which is formed overthe EL layer 616 and functions as a cathode, a material having a lowwork function (e.g., Al, Mg, Li, Ca, or an alloy or a compound thereof,such as MgAg, MgIn, or AlLi) is preferably used. In the case where lightgenerated in the EL layer 616 passes through the second electrode 617, astack of a thin metal film and a transparent conductive film (e.g., ITO,indium oxide containing zinc oxide at 2 wt % to 20 wt %, indium tinoxide containing silicon, or zinc oxide (ZnO)) is preferably used forthe second electrode 617.

Note that the light-emitting element is formed with the first electrode613, the EL layer 616, and the second electrode 617. The light-emittingelement is the light-emitting element described in Embodiments 1 and 2.In the light-emitting device of this embodiment, the pixel portion,which includes a plurality of light-emitting elements, may include boththe light-emitting element described in Embodiments 1 and 2 and alight-emitting element having a different structure.

Further, the sealing substrate 604 is attached to the element substrate610 with the sealing material 605, so that a light-emitting element 618is provided in the space 607 surrounded by the element substrate 610,the sealing substrate 604, and the sealing material 605. The space 607may be filled with filler, and may be filled with an inert gas (such asnitrogen or argon), or the sealing material 605. It is preferable thatthe sealing substrate be provided with a recessed portion and the dryingagent 625 be provided in the recessed portion, in which casedeterioration due to influence of moisture can be suppressed.

An epoxy-based resin or glass frit is preferably used for the sealingmaterial 605. It is preferable that such a material do not transmitmoisture or oxygen as much as possible. As the sealing substrate 604, aglass substrate, a quartz substrate, or a plastic substrate formed offiberglass reinforced plastic (FRP), polyvinyl fluoride) (PVF),polyester, acrylic, or the like can be used.

As described above, the light-emitting device which uses thelight-emitting element described in Embodiments 1 and 2 can be obtained.

The light-emitting device in this embodiment is fabricated using thelight-emitting element described in Embodiments 1 and 2 and thus canhave favorable characteristics. Specifically, since the light-emittingelement described in Embodiments 1 and 2 has high emission efficiency,the light-emitting device can have reduced power consumption. Inaddition, since the light-emitting element has low drive voltage, thelight-emitting device can be driven at low voltage.

Although an active matrix light-emitting device is described in thisembodiment as described above, a passive matrix light-emitting devicemay be manufactured. FIGS. 5A and 5B illustrate a passive matrixlight-emitting device manufactured using the present invention. FIG. 5Ais a perspective view of the light-emitting device, and FIG. 5B is across-sectional view taken along the line X-Y in FIG. 5A. In FIGS. 5Aand 5B, over a substrate 951, an EL layer 955 is provided between anelectrode 952 and an electrode 956. An end portion of the electrode 952is covered with an insulating layer 953. In addition, a partition layer954 is provided over the insulating layer 953. The sidewalls of thepartition layer 954 are aslope such that the distance between bothsidewalls is gradually narrowed toward the surface of the substrate. Inother words, a cross section taken along the direction of the short sideof the partition wall layer 954 is trapezoidal, and the lower side (aside which is in the same direction as a plane direction of theinsulating layer 953 and in contact with the insulating layer 953) isshorter than the upper side (a side which is in the same direction asthe plane direction of the insulating layer 953 and not in contact withthe insulating layer 953). The partition layer 954 thus provided canprevent defects in the light-emitting element due to static electricityor the like. The passive matrix light-emitting device can also be drivenwith low power consumption by including the light-emitting element inEmbodiments 1 and 2 which is capable of operating at low voltage.Further, the light-emitting device can have high reliability byincluding the light-emitting element described in Embodiments 1 and 2.

Further, for performing full color display, a coloring layer or a colorconversion layer may be provided in a light path through which lightfrom the light-emitting element passes to the outside of thelight-emitting device. An example of a light-emitting device in whichfull color display is achieved with the use of a coloring layer and thelike is illustrated in FIGS. 6A and 6B. In FIG. 6A, a substrate 1001, abase insulating film 1002, a gate insulating film 1003, gate electrodes1006, 1007, and 1008, a first interlayer insulating film 1020, a secondinterlayer insulating film 1021, a peripheral portion 1042, a pixelportion 1040, a driver circuit portion 1041, first electrodes 1024W,1024R, 1024G, and 1024B of light-emitting elements, a partition wall1025, an EL layer 1028, a second electrode 1029 of the light-emittingelements, a sealing substrate 1031, and a sealant 1032 are illustrated.Further, coloring layers (a red coloring layer 1034R, a green coloringlayer 1034E and a blue coloring layer 1034B) are provided on atransparent base material 1033. Further, a black layer (a black matrix)1035 may be additionally provided. The transparent base material 1033provided with the coloring layers and the black layer is positioned andfixed to the substrate 1001. Note that the coloring layers and the blacklayer are covered with an overcoat layer 1036. In this embodiment, lightemitted from some of the light-emitting layers does not pass through thecoloring layers, while light emitted from the others of thelight-emitting layers passes through the coloring layers. Since lightwhich does not pass through the coloring layers is white and light whichpasses through any one of the coloring layers is red, blue, or green, animage can be displayed using pixels of the four colors.

The above-described light-emitting device is a light-emitting devicehaving a structure in which light is extracted from the substrate 1001side where the TFTs are formed (a bottom emission structure), but may bea light-emitting device having a structure in which light is extractedfrom the sealing substrate 1031 side (a top emission structure). FIG. 7is a cross-sectional view of a light-emitting device having a topemission structure. In this case, a substrate which does not transmitlight can be used as the substrate 1001. The process up to the step offorming of a connection electrode which connects the TFT and the anodeof the light-emitting element is performed in a manner similar to thatof the light-emitting device having a bottom emission structure. Then, athird interlayer insulating film 1037 is formed to cover an electrode1022. The third interlayer insulating film 1037 may have a planarizationfunction. The third interlayer insulating film 1037 can be formed usinga material similar to that of the second interlayer insulating film, andcan alternatively be formed using any other known material.

The first electrodes 1024W, 1024R, 1024G, and 1024B of thelight-emitting elements each serve as an anode here, but may serve as acathode. Further, in the case of a light-emitting device having a topemission structure as illustrated in FIG. 7, the first electrodes arepreferably reflective electrodes. The EL layer 1028 is formed to have astructure similar to the structure described in Embodiments 1 and 2,with which white light emission can be obtained. As the structure withwhich white light emission can be obtained, in the case where two ELlayers are used, a structure with which blue light is obtained from alight-emitting layer in one of the EL layers and orange light isobtained from a light-emitting layer of the other of the EL layers; astructure in which blue light is obtained from a light-emitting layer ofone of the EL layers and red light and green light are obtained from alight-emitting layer of the other of the EL layers; and the like can begiven. Further, in the case where three EL layers are used, red light,green light, and blue light are obtained from respective light-emittinglayers, so that a light-emitting element which emits white light can beobtained. Needless to say, the structure with which white light emissionis obtained is not limited thereto as long as the structure described inEmbodiments 1 and 2 is used.

The coloring layers are each provided in a light path through whichlight from the light-emitting element passes to the outside of thelight-emitting device. In the case of the light-emitting device having abottom emission structure as illustrated in FIG. 6A, the coloring layers10348, 1034G and 1034B can be provided on the transparent base material1033 and then fixed to the substrate 1001. The coloring layers may beprovided between the gate insulating film 1003 and the first interlayerinsulating film 1020 as illustrated in FIG. 6B. In the case of a topemission structure as illustrated in FIG. 7, sealing can be performedwith the sealing substrate 1031 on which the coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G and the bluecoloring layer 1034B) are provided. The sealing substrate 1031 may beprovided with the black layer (the black matrix) 1035 which ispositioned between pixels. The coloring layers (the red coloring layer1034R, the green coloring layer 1034G and the blue coloring layer 1034B)and the black layer (the black matrix) 1035 may be covered with theovercoat layer 1036. Note that a light-transmitting substrate is used asthe sealing substrate 1031.

When voltage is applied between the pair of electrodes of the thusobtained organic light-emitting element, a white light-emitting region1044W can be obtained. In addition, by using the coloring layers, a redlight-emitting region 1044R, a blue light-emitting region 1044B, and agreen light-emitting region 1044G can be obtained. The light-emittingdevice in this embodiment includes the light-emitting element describedin Embodiments 1 and 2; thus, a light-emitting device with low powerconsumption can be obtained.

Further, although an example in which full color display is performedusing four colors of red, green, blue, and white is shown here, there isno particular limitation and full color display using three colors ofred, green, and blue may be performed.

This embodiment can be freely combined with any of other embodiments.

Embodiment 4

In this embodiment, an example in which the light-emitting elementdescribed in Embodiments 1 and 2 is used for a lighting device will bedescribed with reference to FIGS. 8A and 8B. FIG. 8B is a top view ofthe lighting device, and FIG. 8A is a cross-sectional view taken alongthe line e-f in FIG. 8B.

In the lighting device in this embodiment, a first electrode 401 isformed over a substrate 400 which is a support and has alight-transmitting property. The first electrode 401 corresponds to thefirst electrode 101 in Embodiment 3.

An auxiliary electrode 402 is provided over the first electrode 401.Since light emission is extracted through the first electrode 401 sidein the example given in this embodiment, the first electrode 401 isformed using a material having a light-transmitting property. Theauxiliary electrode 402 is provided in order to compensate for the lowconductivity of the material having a light-transmitting property, andhas a function of suppressing luminance unevenness in a light emissionsurface due to voltage drop caused by the high resistance of the firstelectrode 401. The auxiliary electrode 402 is formed using a materialhaving at least higher conductivity than the material of the firstelectrode 401, and is preferably formed using a material having highconductivity such as aluminum. Note that surfaces of the auxiliaryelectrode 402 other than a portion thereof in contact with the firstelectrode 401 are preferably covered with an insulating layer. This isfor suppressing light emission over the upper portion of the auxiliaryelectrode 402, which cannot be extracted, for reducing a reactivecurrent, and for suppressing a reduction in power efficiency. Note thata pad 412 for applying a voltage to a second electrode 404 may be formedat the same time as the formation of the auxiliary electrode 402.

An EL layer 403 is formed over the first electrode 401 and the auxiliaryelectrode 402. The EL layer 403 has the structure described inEmbodiments 1 and 2. Refer to the descriptions for the structure. Notethat the EL layer 403 is preferably formed to be slightly larger thanthe first electrode 401 when seen from above, in which case the EL layer403 can also serve as an insulating layer that suppresses a shortcircuit between the first electrode 401 and the second electrode 404.

The second electrode 404 is formed to cover the EL layer 403. The secondelectrode 404 corresponds to the second electrode 102 in Embodiment 3and has a similar structure. In this embodiment, it is preferable thatthe second electrode 404 be formed using a material having highreflectance because light emission is extracted through the firstelectrode 401 side. In this embodiment, the second electrode 404 isconnected to the pad 412, whereby voltage is applied.

As described above, the lighting device described in this embodimentincludes a light-emitting element including the first electrode 401, theEL layer 403, and the second electrode 404 (and the auxiliary electrode402). Since the light-emitting element is a light-emitting element withhigh emission efficiency, the lighting device in this embodiment can bea lighting device having low power consumption. Furthermore, since thelight-emitting element is a light-emitting element having highreliability, the lighting device in this embodiment can be a lightingdevice having high reliability.

The light-emitting element having the above structure is fixed to asealing substrate 407 with sealing materials 405 and 406 and sealing isperformed, whereby the lighting device is completed. It is possible touse only either the sealing material 405 or the sealing material 406. Inaddition, the inner sealing material 406 can be mixed with a desiccantwhich enables moisture to be adsorbed, increasing reliability.

When parts of the pad 412, the first electrode 401, and the auxiliaryelectrode 402 are extended to the outside of the sealing materials 405and 406, the extended parts can serve as external input terminals. An ICchip 420 mounted with a converter or the like may be provided over theexternal input terminals.

As described above, since the lighting device described in thisembodiment includes the light-emitting element described in Embodiments1 and 2 as an EL element, the lighting device can be a lighting devicehaving low power consumption. Further, the lighting device can be alighting device having low drive voltage. Furthermore, the lightingdevice can be a lighting device having high reliability.

Embodiment 5

In this embodiment, examples of electronic devices each including thelight-emitting element described in Embodiments 1 and 2 will bedescribed. The light-emitting element described in Embodiments 1 and 2has high emission efficiency and reduced power consumption. As a result,the electronic devices described in this embodiment can each include alight-emitting portion having reduced power consumption. In addition,the electronic devices can be driven at low voltage since thelight-emitting element described in Embodiments 1 and 2 has low drivevoltage.

Examples of the electronic device to which the above light-emittingelement is applied include television devices (also referred to as TV ortelevision receivers), monitors for computers and the like, cameras suchas digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as cell phones or mobile phone devices),portable game machines, portable information terminals, audio playbackdevices, large game machines such as pachinko machines, and the like.Specific examples of these electronic devices are given below.

FIG. 9A illustrates an example of a television device. In the televisiondevice, a display portion 7103 is incorporated in a housing 7101. Inaddition, here, the housing 7101 is supported by a stand 7105. Imagescan be displayed on the display portion 7103, and in the light-emittingportion 7103, the light-emitting elements described in Embodiments 1 and2 are arranged in a matrix. The light-emitting elements can have highemission efficiency. Further, the light-emitting elements can be drivenat low voltage. Furthermore, the light-emitting elements can have a longlifetime. Therefore, the television device including the display portion7103 which is formed using the light-emitting elements can be atelevision device having reduced power consumption. Further, thetelevision device can be a television device having low drive voltage.Furthermore, the television device can be a television device havinghigh reliability.

Operation of the television device can be performed with an operationswitch of the housing 7101 or a separate remote controller 7110. Withoperation keys 7109 of the remote controller 7110, channels and volumecan be controlled and images displayed on the display portion 7103 canbe controlled. Furthermore, the remote controller 7110 may be providedwith a display portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 9B1 illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured by using light-emitting elements arrangedin a matrix in the display portion 7203, which are the same as thatdescribed in Embodiment 2 or 3. The computer illustrated in FIG. 9B1 mayhave a structure illustrated in FIG. 9B2. The computer illustrated inFIG. 9B2 is provided with a second display portion 7210 instead of thekeyboard 7204 and the pointing device 7206. The second display portion7210 is a touch screen, and input can be performed by operation ofdisplay for input on the second display portion 7210 with a finger or adedicated pen. The second display portion 7210 can also display imagesother than the display for input. The display portion 7203 may be also atouch screen. Connecting the two screens with a hinge can preventtroubles; for example, the screens can be prevented from being crackedor broken while the computer is being stored or carried. Thelight-emitting elements can have high emission efficiency. Therefore,this computer having the display portion 7203 which is formed using thelight-emitting elements consumes less power.

FIG. 9C illustrates a portable game machine having two housings, ahousing 7301 and a housing 7302, which are connected with a jointportion 7303 so that the portable game machine can be opened or folded.The housing 7301 incorporates a display portion 7304 including thelight-emitting elements described in Embodiments 1 and 2 and arranged ina matrix, and the housing 7302 incorporates a display portion 7305. Inaddition, the portable game machine illustrated in FIG. 9C includes aspeaker portion 7306, a recording medium insertion portion 7307, an LEDlamp 7308, an input means (an operation key 7309, a connection terminal7310, a sensor 7311 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared rays), and a microphone 7312), and thelike. Needless to say, the structure of the portable game machine is notlimited to the above as long as the display portion which includes thelight-emitting elements described in Embodiments 1 and 2 and arranged ina matrix is used as at least either the display portion 7304 or thedisplay portion 7305, or both, and the structure can include otheraccessories as appropriate. The portable game machine illustrated inFIG. 9C has a function of reading out a program or data stored in astorage medium to display it on the display portion, and a function ofsharing information with another portable game machine by wirelesscommunication. Note that functions of the portable game machineillustrated in FIG. 9C are not limited to them, and the portable gamemachine can have various functions. Since the light-emitting elementsused in the display portion 7304 have high emission efficiency, theportable game machine including the above-described display portion 7304can be a portable game machine having reduced power consumption. Sincethe light-emitting elements used in the display portion 7304 each can bedriven at low voltage, the portable game machine can also be a portablegame machine having low drive voltage. Furthermore, since thelight-emitting elements used in the display portion 7304 each have along lifetime, the portable game machine can be highly reliable.

FIG. 9D illustrates an example of a mobile phone. The mobile phone isprovided with a display portion 7402 incorporated in a housing 7401,operation buttons 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400has the display portion 7402 including the light-emitting elementsdescribed in Embodiments 1 and 2 and arranged in a matrix. Thelight-emitting elements can have high emission efficiency. Further, thelight-emitting elements can be driven at low voltage. Furthermore, thelight-emitting elements can have a long lifetime. Therefore, the mobilephone including the display portion 7402 which is formed using thelight-emitting elements can be a mobile phone having reduced powerconsumption. Further, the mobile phone can be a mobile phone having lowdrive voltage. Furthermore, the mobile phone can be a mobile phonehaving high reliability.

When the display portion 7402 of the mobile phone illustrated in FIG. 9Dis touched with a finger or the like, data can be input into the mobilephone. In this case, operations such as making a call and creating ane-mail can be performed by touching the display portion 7402 with afinger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting information such ascharacters. The third mode is a display-and-input mode in which twomodes of the display mode and the input mode are combined.

For example, in the case of making a call or creating an e-mail, acharacter input mode mainly for inputting characters is selected for thedisplay portion 7402 so that characters displayed on a screen can beinput. In this case, it is preferable to display a keyboard or numberbuttons on almost the entire screen of the display portion 7402.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone, display on the screen of the display portion 7402 can beautomatically changed by determining the orientation of the mobile phone(whether the mobile phone is placed horizontally or vertically for alandscape mode or a portrait mode).

The screen modes are switched by touch on the display portion 7402 oroperation with the operation buttons 7403 of the housing 7401. Thescreen modes can be switched depending on the kind of images displayedon the display portion 7402. For example, when a signal of an imagedisplayed on the display portion is a signal of moving image data, thescreen mode is switched to the display mode. When the signal is a signalof text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion7402 is not performed for a certain period while a signal detected by anoptical sensor in the display portion 7402 is detected, the screen modemay be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal authentication can be performed. Further, by providing abacklight or a sensing light source which emits a near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

Note that the structure described in this embodiment can be combinedwith any of the structures described in Embodiments 1 to 4 asappropriate.

As described above, the application range of the light-emitting devicehaving the light-emitting element described in Embodiments 1 and 2 iswide so that this light-emitting device can be applied to electronicdevices in a variety of fields. By using the light-emitting elementdescribed in Embodiments 1 and 2, an electronic device having reducedpower consumption can be obtained.

FIG. 10 illustrates an example of a liquid crystal display device usingthe light-emitting element described in Embodiments 1 and 2 for abacklight. The liquid crystal display device illustrated in FIG. 10includes a housing 901, a liquid crystal layer 902, a backlight unit903, and a housing 904. The liquid crystal layer 902 is connected to adriver IC 905. The light-emitting element described in Embodiments 1 and2 is used in the backlight unit 903, to which current is suppliedthrough a terminal 906.

The light-emitting element described in Embodiments 1 and 2 is used forthe backlight of the liquid crystal display device; thus, the backlightcan have reduced power consumption. In addition, the use of thelight-emitting element described in Embodiment 2 enables manufacture ofa planar-emission lighting device and further a larger-areaplanar-emission lighting device; therefore, the backlight can be alarger-area backlight, and the liquid crystal display device can also bea larger-area device. Furthermore, the light-emitting device using thelight-emitting element described in Embodiment 2 can be thinner than aconventional one; accordingly, the display device can also be thinner.

FIG. 11 illustrates an example in which the light-emitting elementdescribed in Embodiments 1 and 2 is used for a table lamp which is alighting device. The table lamp illustrated in FIG. 11 includes ahousing 2001 and a light source 2002, and the light-emitting devicedescribed in Embodiment 4 is used for the light source 2002.

FIG. 12 illustrates an example in which the light-emitting elementdescribed in Embodiments 1 and 2 is used for an indoor lighting device3001 and a display device 3002. Since the light-emitting elementdescribed in Embodiments 1 and 2 has reduced power consumption, alighting device that has reduced power consumption can be obtained.Further, since the light-emitting element described in Embodiments 1 and2 can have a large area, the light-emitting element can be used for alarge-area lighting device. Furthermore, since the light-emittingelement described in Embodiments 1 and 2 is thin, the light-emittingelement can be used for a lighting device having a reduced thickness.

The light-emitting element described in Embodiments 1 and 2 can also beused for an automobile windshield or an automobile dashboard. FIG. 13illustrates one mode in which the light-emitting elements described inEmbodiment 2 are used for an automobile windshield and an automobiledashboard. Displays 5000 to 5005 each include the light-emitting elementdescribed in Embodiments 1 and 2.

The display 5000 and the display 5001 are display devices which areprovided in the automobile windshield and in which the light-emittingelements described in Embodiments 1 and 2 are incorporated. Thelight-emitting element described in Embodiments 1 and 2 can be formedinto what is called a see-through display device, through which theopposite side can be seen, by including a first electrode and a secondelectrode formed of electrodes having light-transmitting properties.Such see-through display devices can be provided even in the windshieldof the car, without hindering the vision. Note that in the case where atransistor for driving is provided, a transistor having alight-transmitting property, such as an organic transistor using anorganic semiconductor material or a transistor using an oxidesemiconductor, is preferably used.

The display 5002 is a display device which is provided in a pillarportion and in which the light-emitting elements described inEmbodiments 1 and 2 are incorporated. The display 5002 can compensatefor the view hindered by the pillar portion by showing an image taken byan imaging unit provided in the car body. Similarly, the display 5003provided in the dashboard can compensate for the view hindered by thecar body by showing an image taken by an imaging unit provided in theoutside of the car body, which leads to elimination of blind areas andenhancement of safety. Showing an image so as to compensate for the areawhich a driver cannot see makes it possible for the driver to confirmsafety easily and comfortably.

The display 5004 and the display 5005 can provide a variety of kinds ofinformation such as navigation data, a speedometer, a tachometer, amileage, a fuel meter, a gearshift indicator, and air-condition setting.The content or layout of the display can be changed freely by a user asappropriate. Note that such information can also be shown by thedisplays 5000 to 5003. The displays 5000 to 5005 can also be used aslighting devices.

The light-emitting element described in Embodiments 1 and 2 can havehigh emission efficiency and low power consumption. Therefore, load on abattery is small even when a number of large screens such as thedisplays 5000 to 5005 are provided, which provides comfortable use. Forthat reason, the light-emitting device and the lighting device each ofwhich includes the light-emitting element described in Embodiments 1 and2 can be suitably used as an in-vehicle light-emitting device and anin-vehicle lighting device.

FIGS. 14A and 14B illustrate an example of a foldable tablet terminal.FIG. 14A illustrates the tablet terminal which is unfolded. The tabletterminal includes a housing 9630, a display portion 9631 a, a displayportion 9631 b, a display mode switch 9034, a power switch 9035, apower-saving mode switch 9036, a clasp 9033, and an operation switch9038. Note that in the tablet terminal, one or both of the displayportion 9631 a and the display portion 9631 b is/are formed using alight-emitting device which includes the light-emitting elementdescribed in Embodiments 1 and 2.

Part of the display portion 9631 a can be a touchscreen region 9632 aand data can be input when a displayed operation key 9637 is touched.Although half of the display portion 9631 a has only a display functionand the other half has a touchscreen function, one embodiment of thepresent invention is not limited to the structure. The whole displayportion 9631 a may have a touchscreen function. For example, a keyboardis displayed on the entire region of the display portion 9631 a so thatthe display portion 9631 a is used as a touchscreen; thus, the displayportion 9631 b can be used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touchscreen region 9632 b. When a switching button 9639 forshowing/hiding a keyboard on the touchscreen is touched with a finger, astylus, or the like, the keyboard can be displayed on the displayportion 9631 b.

Touch input can be performed in the touchscreen region 9632 a and thetouchscreen region 9632 b at the same time.

The display mode switch 9034 can switch the display between portraitmode, landscape mode, and the like, and between monochrome display andcolor display, for example. The power-saving switch 9036 can controldisplay luminance in accordance with the amount of external light in useof the tablet terminal detected by an optical sensor incorporated in thetablet terminal. Another detection device including a sensor fordetecting inclination, such as a gyroscope or an acceleration sensor,may be incorporated in the tablet terminal, in addition to the opticalsensor.

Although FIG. 14A illustrates an example in which the display portion9631 a and the display portion 9631 b have the same display area, oneembodiment of the present invention is not limited to the example. Thedisplay portion 9631 a and the display portion 9631 b may have differentdisplay areas and different display quality. For example, higherdefinition images may be displayed on one of the display portions 9631 aand 9631 b.

FIG. 14B illustrates the tablet terminal which is folded. The tabletterminal in this embodiment includes the housing 9630, a solar cell9633, a charge and discharge control circuit 9634, a battery 9635, and aDC-to-DC converter 9636. As an example, FIG. 14B illustrates the chargeand discharge control circuit 9634 including the battery 9635 and theDC-to-DC converter 9636.

Since the tablet terminal is foldable, the housing 9630 can be closedwhen the tablet terminal is not in use. As a result, the display portion9631 a and the display portion 9631 b can be protected, therebyproviding a tablet terminal with high endurance and high reliability forlong-term use.

The tablet terminal illustrated in FIGS. 14A and 14B can have otherfunctions such as a function of displaying various kinds of data (e.g.,a still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a touch-input function of operating or editing the datadisplayed on the display portion by touch input, and a function ofcontrolling processing by various kinds of software (programs).

The solar cell 9633 provided on a surface of the tablet terminal cansupply power to the touchscreen, the display portion, a video signalprocessing portion, or the like. Note that the solar cell 9633 can beprovided on one or both surfaces of the housing 9630, so that thebattery 9635 can be charged efficiently.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 14B will be described with reference to a blockdiagram of FIG. 14C. FIG. 14C illustrates the solar cell 9633, thebattery 9635, the DC-to-DC converter 9636, a converter 9638, switchesSW1 to SW3, and the display portion 9631. The battery 9635, the DC-to-DCconverter 9636, the converter 9638, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 illustratedin FIG. 14B.

First, description is made on an example of the operation in the casewhere power is generated by the solar cell 9633 with the use of externallight. The voltage of the power generated by the solar cell is raised orlowered by the DC-to-DC converter 9636 so as to be voltage for chargingthe battery 9635. Then, when power from the solar cell 9633 is used forthe operation of the display portion 9631, the switch SW1 is turned onand the voltage of the power is raised or lowered by the converter 9638so as to be voltage needed for the display portion 9631. When images arenot displayed on the display portion 9631, the switch SW1 is turned offand the switch SW2 is turned on so that the battery 9635 is charged.

Although the solar cell 9633 is described as an example of a powergeneration means, the power generation means is not particularlylimited, and the battery 9635 may be charged by another power generationmeans such as a piezoelectric element or a thermoelectric conversionelement (Peltier element). The battery 9635 may be charged by anon-contact power transmission module capable of performing charging bytransmitting and receiving power wirelessly (without contact), or any ofthe other charge means used in combination, and the power generationmeans is not necessarily provided.

One embodiment of the present invention is not limited to the tabletterminal having the shape illustrated in FIGS. 14A to 14C as long as thedisplay portion 9631 is included.

Example 1

In this example, a method for fabricating a light-emitting element whichcorresponds to one embodiment of the present invention described inEmbodiment 1 and Embodiment 2 and the characteristics thereof aredescribed. Structural formulae of organic compounds used in this exampleare shown below.

Next, a method for fabricating the light-emitting element in thisexample is described below.

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness thereof was 110 nm and theelectrode area was 2 mm×2 mm. Here, the first electrode 101 is anelectrode that functions as an anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for about 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. The pressure in the vacuum evaporation apparatus was reducedto about 10⁻⁴ Pa. After that, over the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (1) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating, so that the hole-injection layer 111 was formed. Thethickness of the hole-injection layer 111 was set to 33 nm, and theweight ratio of DBT3P-II to molybdenum oxide was adjusted to 4:2. Notethat the co-evaporation method refers to an evaporation method in whichevaporation is carried out from a plurality of evaporation sources atthe same time in one treatment chamber.

Next, a film of 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP) which is represented by Structural Formula (ii)was formed to a thickness of 20 nm over the hole-injection layer 111 toform the hole-transport layer 112.

Further, over the hole-transport layer 112,2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II) represented by Structural Formula (iii),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP) represented by Structural Formula (Iv), andbis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)) represented by Structural Formula (v)were deposited by co-evaporation to a thickness of 20 nm with a massratio of 2mDBTPDBq-II to PCBA1BP and Ir(tBuppm)₂(acac) being0.8:0.2:0.05, so that the first light-emitting layer 113 a was formed;then, 2mDBTPDBq-II andbis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: Ir(tppr)₂(dpm)) represented by Structural Formula (vi)were deposited by co-evaporation to a thickness of 20 nm with a massratio of 2mDBTPDBq-II to Ir(tppr)₂(dpm) being 1:0.06, so that the secondlight-emitting layer 113 b was formed. Note that 2mDBTPDBq-II, which isa host material, and PCBA1BP form an exciplex.

Then, the electron-transport layer 114 was formed over thelight-emitting layer 113 in such a way that a 15-nm-thick film of2mDBTPDBq-II was formed and a 15-nm-thick film of bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (vii) wasformed.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm, sothat the electron-injection layer 115 was formed.

Lastly, aluminum was deposited by evaporation to a thickness of 200 nmto form the second electrode 102 functioning as a cathode. Thus, alight-emitting element 1 in this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 1 shows an element structure of the light-emitting element 1obtained as described above.

TABLE 1 Hole- Light-emitting Layer Electron- Hole-injection transportFirst Light-emitting Second Light-emitting injection Layer Layer LayerLayer Electron-transport Layer Layer DBT3P-II:MoO_(x) BPAFLP2mDBTPDBq-II:PCBA1BP:Ir(tBuPPm)₂(acac) 2mDBTPDBq-II:Ir(tppr)₂(dpm)2mDBTPDBq- Bphen LiF 4:2 20 nm 0.8:0.2:0.05 1:0.06 II 15 nm 1 nm 33 nm20 nm 20 nm 15 nm

The light-emitting element 1 was sealed using a glass substrate in aglove box containing a nitrogen atmosphere so as not to be exposed tothe air (specifically, a sealing material was applied onto an outer edgeof the element and heat treatment was performed at 80° C. for 1 hour atthe time of sealing).

In the light-emitting element 1, Ir(tBuppm)₂(acac) and Ir(tppr)₂(dpm)were used as the first phosphorescent compound 113Da and the secondphosphorescent compound 113Db, respectively. Here, a relation between aPL spectrum of Ir(tBuppm)₂(acac) and ε(λ)λ⁴ of Ir(tppr)₂(dpm) isdescribed. Note that λ denotes a wavelength and ε(λ) denotes a molarabsorption coefficient.

FIG. 20A shows graphs of the molar absorption coefficient ε(λ) andε(λ)λ⁴ of Ir(tppr)₂(dpm). While the molar absorption coefficient ε(λ)does not have a noticeable peak in a region on a longer wavelength side,the graph of ε(λ)λ⁴ has a peak including a local maximum value at 543nm. This peak shows triplet MLCT absorption of Ir(tppr)₂(dpm). When thispeak has an overlap with an emission peak of the first phosphorescentcompound 113Da, energy transfer efficiency can be largely increased.

FIG. 20B shows the PL spectrum F(λ) of Ir(tBuppm)₂(acac) which is thefirst phosphorescent compound 113Da and the graph of ε(λ)λ⁴ ofIr(tppr)₂(dpm) which is the second phosphorescent compound 113Db. Asseen from the graph, a band having a peak of the PL spectrum F(λ) ofIr(tBuppm)₂(acac) largely overlaps with the band having thelongest-wavelength-side peak of ε(λ)λ⁴ of Ir(tppr)₂(dpm), whichindicates that the combination enables extremely efficient energytransfer. Further, Ir(tBuppm)₂(acac) which is the first phosphorescentcompound 113Da has an emission peak at 546 nm, and the spectrum showingε(λ)λ⁴ of Ir(tppr)₂(dpm) which is the second phosphorescent compound113Db has a longer-wavelength-side local maximum at 543 nm, so that thedifference is 3 nm. The wavelengths 546 nm and 543 nm correspond to 2.27eV and 2.28 eV, respectively, so that the difference is 0.01 eV, whichis less than 0.2 eV; thus, the positions of the peaks also suggestoccurrence of efficient energy transfer.

Next, FIG. 21A shows graphs of the molar absorption coefficient ε(λ) andε(λ)λ⁴ of Ir(tBuppm)₂(acac) which is the first phosphorescent compound113Da. While a peak in a region on the longer wavelength side has alower intensity than a peak on the shorter wavelength side in the graphof the molar absorption coefficient ε(λ), the graph of ε(λ)λ⁴ has a peakwith a high intensity at 494 nm. The peak which has this peak showstriplet MLCT absorption of Ir(tBuppm)₂(acac). When this peak has anoverlap with an emission peak of an energy donor, energy transferefficiency can be largely increased.

Here, in the light-emitting element 1 in this example, 2mDBTPDBq-IIwhich is the first host material and PCBA1BP which is the first organiccompound form the exciplex 113Ec, and energy is transferred from theexciplex 113Ec to the first phosphorescent compound 113Da. FIG. 23 showsPL spectra of 2mDBTPDBq-II, PCBA1BP, and a mixed film thereof (a massratio of 2mDBTPDBq-II to PCBA1BP is 0.8:0.2), and it can be found that2mDBTPDBq-II and PCBA1BP which is the first organic compound formed theexciplex 113Ec. FIG. 21B shows a PL spectrum F(λ) of the exciplex andthe graph of ε(λ)λ⁴ of Ir(tBuppm)₂(acac) which is the firstphosphorescent compound 113Da. As seen from the graph, a band having apeak of the PL spectrum F(λ) of the exciplex overlaps with the bandhaving the longest-wavelength-side peak of ε(λ)λ⁴ of Ir(tBuppm)₂(acac),which indicates that the combination enables efficient energy transfer.Further, the PL spectrum of the exciplex has a peak at 519 nm, and thespectrum showing ε(λ)λ⁴ of Ir(tBuppm)₂(acac) which is the firstphosphorescent compound 113Da has a longer-wavelength-side local maximumat 494 nm, so that the difference is 25 nm. The wavelengths 519 nm and494 nm respectively correspond to 2.39 eV and 2.51 eV when convertedinto energy, so that the difference is 0.12 eV, which is less than 0.2eV; thus, the positions of the peaks also suggest occurrence ofefficient energy transfer.

Note that as can be seen from FIG. 23, the PL spectrum of 2mDBTPDBq-IIwhich is the first host material 113Ha has a peak at 426 nm, whichcorresponds to 2.91 eV when converted into energy. Further, the PLspectrum of PCBA1BP which is the first organic compound 113A has a peakat 405 nm, which corresponds to 3.06 eV when converted into energy. Thespectrum showing ε(λ)λ⁴ of Ir(tBuppm)₂(acac) has thelonger-wavelength-side peak at 494 nm, which corresponds to 2.51 eV whenconverted into energy. Therefore, the difference with 2mDBTPDBq-II thatis the first host material 113Ha is 0.4 eV and the difference withPCBA1BP that is the first organic compound 113A is 0.55 eV, each ofwhich exceeds 0.2 eV; thus, it can be found that energy is not readilytransferred from 2mDBTPDBq-II or PCBA1BP to Ir(tBuppm)₂(acac).

FIG. 22 shows the PL spectrum F(λ) of the exciplex, the PL spectrum F(λ)of Ir(tBuppm)₂(acac), a PL spectrum F(λ) of Ir(tppr)₂(dpm), ε(λ)λ⁴ ofIr(tBuppm)₂(acac), and ε(λ)λ⁴ of Ir(tppr)₂(dpm). It can be found thatenergy can be transferred stepwise first from the exciplex toIr(tBuppm)₂(acac) by utilizing the overlap between the PL spectrum ofthe exciplex and ε(λ)λ⁴ of Ir(tBuppm)₂(acac) (around a peak A), and thenfrom Ir(tBuppm)₂(acac) to Ir(tppr)₂(dpm) by utilizing the overlapbetween the PL spectrum of Ir(tBuppm)₂(acac) and ε(λ)λ⁴ ofIr(tppr)₂(dpm) (around a peak B). Note that direct energy transfer fromthe exciplex to Ir(tppr)₂(dpm) which is the second phosphorescentcompound is also possible. The reason for this is that, as can be seenfrom FIG. 22, ε(λ)λ⁴ of Ir(tppr)₂(dpm) also overlaps with the PLspectrum F(λ) of the exciplex on a shorter wavelength side of thetriplet MLCT absorption band (around the peak B) of Ir(tppr)₂(dpm).

Element characteristics of the light-emitting element were measured.Note that the measurement was carried out at room temperature (in anatmosphere kept at 25° C.).

FIG. 15 shows luminance-current efficiency characteristics of thelight-emitting element 1. In FIG. 15, the horizontal axis indicatesluminance (cd/m²), and the vertical axis indicates current efficiency(cd/A). FIG. 16 shows voltage-luminance characteristics thereof. In FIG.16, the horizontal axis indicates voltage (V) and the vertical axisindicates luminance (cd/m²). FIG. 17 shows luminance-external quantumefficiency characteristics thereof. In FIG. 17, the horizontal axisindicates luminance (cd/m²) and the vertical axis indicates externalquantum efficiency (%). FIG. 18 shows luminance-power efficiencycharacteristics thereof. In FIG. 18, the horizontal axis indicatesluminance (cd/m²) and the vertical axis indicates power efficiency(lm/w).

From the above, the light-emitting element 1 turned out to haveexcellent element characteristics. In particular, as can be seen fromFIG. 15, FIG. 17, and FIG. 18, the light-emitting element has extremelyhigh emission efficiency and had a high external quantum efficiency notless than 20% at around a practical luminance (1000 cd/m²). In addition,the current efficiency was around 60 cd/A and the power efficiency wasaround 60 lm/W, which are excellent values.

FIG. 19 shows an emission spectrum of the light-emitting element 1 whichwas obtained when a current of 0.1 mA was made to flow in thelight-emitting element 1. In FIG. 19, the horizontal axis indicates awavelength (nm) and the vertical axis indicates light emission intensity(arbitrary unit). FIG. 19 indicates that the light-emitting element 1shows an emission spectrum including light with a wavelength in a greenwavelength range which originates frombis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)) and light with a wavelength in a redwavelength range which originates frombis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: Ir(tppr)₂(dpm)) in a good balance.

From the above, it was shown that the light-emitting element 1corresponding to one embodiment of the present invention has highemission efficiency and provides lights from two kinds of emissioncenter substances in a good balance.

Example 2

In this example, a method for fabricating a light-emitting element whichcorresponds to one embodiment of the present invention described inEmbodiment 1 and Embodiment 2 and the characteristics thereof aredescribed. Structural formulae of organic compounds used in this exampleare shown below.

Next, a method for fabricating the light-emitting element in thisexample is described below.

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness thereof was 110 nm and theelectrode area was 2 mm×2 mm. Here, the first electrode 101 is anelectrode that functions as an anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for about 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. The pressure in the vacuum evaporation apparatus was reducedto about 10⁻⁴ Pa. After that, over the first electrode 101,3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPPn)represented by Structural Formula (viii) and molybdenum(VI) oxide weredeposited by co-evaporation by an evaporation method using resistanceheating, so that the hole-injection layer 111 was formed. The thicknessof the hole-injection layer 111 was set to 33.3 nm, and the weight ratioof PCPPn to molybdenum oxide was adjusted to 1:0.5. Note that theco-evaporation method refers to an evaporation method in whichevaporation is carried out from a plurality of evaporation sources atthe same time in one treatment chamber.

Next, a film of 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP) which is represented by Structural Formula (ii)was formed to a thickness of 20 nm over the hole-injection layer 111 toform the hole-transport layer 112.

Further, over the hole-transport layer 112,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (ix),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB) represented by Structural Formula (x), andbis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)) represented by Structural Formula (v)were deposited by co-evaporation to a thickness of 20 nm with a massratio of 2mDBTBPDBq-II to PCBNBB and Ir(tBuppm)₂(acac) being0.8:0.2:0.06, so that the first light-emitting layer 113 a was formed;then, 2mDBTBPDBq-II andbis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: Ir(tppr)₂(dpm)) represented by Structural Formula (vi)were deposited by co-evaporation to a thickness of 20 nm with a massratio of 2mDBTBPDBq-II to Ir(tppr)₂(dpm) being 1:0.06, so that thesecond light-emitting layer 113 b was formed. Note that 2mDBTBPDBq-II,which is a host material, and PCBNBB form an exciplex.

Then, the electron-transport layer 114 was formed over thelight-emitting layer 113 in such a way that a 15-nm-thick film of2mDBTBPDBq-II was formed and a 15-nm-thick film of bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (vii) wasformed.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm, sothat the electron-injection layer 115 was formed.

Lastly, aluminum was deposited by evaporation to a thickness of 200 nmto form the second electrode 102 functioning as a cathode. Thus, alight-emitting element 2 in this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 2 shows an element structure of the light-emitting element 2obtained as described above.

TABLE 2 Light-emitting Layer Hole-injection Hole-transport FirstLight-emitting Second Electron- Layer Layer Layer Light-emitting LayerElectron-transport Layer injection Layer PCPPn:MoO_(x) BPAFLP2mDBTBPDBq-II:PCBNBB:Ir(tBuppm)₂(acac) 2mDBTBPDBq- 2mDBTBPDBq-II BphenLiF 4:2 20 nm 0.8:0.2:0.06 II:Ir(tppr)₂(dpm) 15 nm 15 nm 1 nm 33 nm 20nm 1:0.06 20 nm

The light-emitting element 2 was sealed using a glass substrate in aglove box containing a nitrogen atmosphere so as not to be exposed tothe air (specifically, a sealing material was applied onto an outer edgeof the element and heat treatment was performed at 80° C. for 1 hour atthe time of sealing).

In the light-emitting element 2, as in the light-emitting element 1,Ir(tBuppm)₂(acac) and Ir(tppr)₂(dpm) were used as the firstphosphorescent compound 113Da and the second phosphorescent compound113Db, respectively. Thus, the relation between the PL spectrum ofIr(tBuppm)₂(acac) and ε(λ)λ⁴ of Ir(tppr)₂(dpm) is the same as in thecase of the light-emitting element 1, and the description thereof is notrepeated. The description made with reference to FIGS. 20A and 20B inExample 1 is to be referred to. Accordingly, it is suggested that energytransfer efficiently occurs between the first phosphorescent compound113Da and the second phosphorescent compound 113Db in the light-emittingelement 2.

Element characteristics of the light-emitting element were measured.Note that the measurement was carried out at room temperature (in anatmosphere kept at 25° C.).

FIG. 24 shows luminance-current efficiency characteristics of thelight-emitting element 2. FIG. 25 shows voltage-luminancecharacteristics thereof. FIG. 26 shows luminance-external quantumefficiency characteristics thereof. FIG. 27 shows luminance-powerefficiency characteristics thereof.

From the above, the light-emitting element 2 turned out to haveexcellent element characteristics. In particular, as can be seen fromFIG. 24, FIG. 26, and FIG. 27, the light-emitting element has extremelyhigh emission efficiency and had a high external quantum efficiency notless than 20% at around a practical luminance (1000 cd/m²). In addition,the current efficiency was around 60 cd/A and the power efficiency wasaround 60 lm/W, which are excellent values.

FIG. 28 shows an emission spectrum of the light-emitting element 2 whichwas obtained when a current of 0.1 mA was made to flow in thelight-emitting element 2. FIG. 28 indicates that the light-emittingelement 2 shows an emission spectrum including light with a wavelengthin a green wavelength range which originates frombis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)) and light with a wavelength in a redwavelength range which originates frombis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: Ir(tppr)₂(dpm)) in a good balance.

Further, FIG. 29 shows the results of a reliability test underconditions where the initial luminance of 5000 cd/m² was taken as 100%,and the current density was constant. As seen in FIG. 29, despite thereliability test with the initial luminance of 5000 cd/m², thelight-emitting element 2 kept 96% of the initial luminance after 70hours elapsed; thus, it was revealed that the light-emitting element hashigh reliability.

From the above, it was shown that the light-emitting element 2corresponding to one embodiment of the present invention has highemission efficiency and provides lights from two kinds of emissioncenter substances in a good balance. It was also shown that thelight-emitting element has high reliability and a long lifetime.

Example 3

In this example, a method for fabricating a light-emitting element whichcorresponds to one embodiment of the present invention described inEmbodiment 1 and Embodiment 2 and the characteristics thereof aredescribed. Structural formulae of organic compounds used in this exampleare shown below.

Next, a method for fabricating the light-emitting element in thisexample is described below.

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness thereof was 110 nm and theelectrode area was 2 mm×2 mm. Here, the first electrode 101 is anelectrode that functions as an anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for about 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. The pressure in the vacuum evaporation apparatus was reducedto about 10⁻⁴ Pa. After that, over the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating, so that the hole-injection layer 111 was formed. Thethickness of the hole-injection layer 111 was set to 40 nm, and theweight ratio of DBT3P-II to molybdenum oxide was adjusted to 4:2. Notethat the co-evaporation method refers to an evaporation method in whichevaporation is carried out from a plurality of evaporation sources atthe same time in one treatment chamber.

Next, a film of 4,4′,4″-tri(N-carbazolyl)triphenylamine (abbreviation:TCTA) which is represented by Structural Formula (xi) was formed to athickness of 10 nm over the hole-injection layer 111 to form thehole-transport layer 112.

Further, over the hole-transport layer 112, TCTA andbis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: Ir(tppr)₂(dpm)) represented by Structural Formula (vi)were deposited by co-evaporation to a thickness of 10 nm with a massratio of TCTA to Ir(tppr)₂(dpm) being 1:0.1, so that the secondlight-emitting layer 113 b was formed; then,2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II) represented by Structural Formula (iii),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP) represented by Structural Formula (iv), andbis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)) represented by Structural Formula (v)were deposited by co-evaporation to a thickness of 5 nm with a massratio of 2mDBTPDBq-II to PCBA1BP and Ir(tBuppm)₂(acac) being0.8:0.2:0.05, so that the first light-emitting layer 113 a was formed.Note that 2mDBTPDBq-II, which is a host material, and PCBA1BP form anexciplex.

Then, the electron-transport layer 114 was formed over thelight-emitting layer 113 in such a way that 2mDBTPDBq-II andIr(tBuppm)₂(acac) were deposited by co-evaporation to a thickness of 20nm with a mass ratio of 2mDBTPDBq-II to Ir(tBuppm)₂(acac) being 1:0.05,a film of 2mDBTPDBq-II was formed to a thickness of 10 nm, and a film ofbathophenanthroline (abbreviation: BPhen) represented by StructuralFormula (vii) was then formed to a thickness of 20 nm.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm, sothat the electron-injection layer 115 was formed.

Lastly, aluminum was deposited by evaporation to a thickness of 200 nmto form the second electrode 102 functioning as a cathode. Thus, alight-emitting element 3 in this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 3 shows an element structure of the light-emitting element 3obtained as described above. The materials in the hole-transport layerand the second host material of the light-emitting element 3 are verydifferent from those of the light-emitting element 1 and thelight-emitting element 2. In addition, there are also big differences inthe positions of the first and second light-emitting layers with respectto the electrode and the structure of the electron-transport layer.

TABLE 3 Hole- Light-emitting Layer Electron- Hole-injection transportSecond Light- First Light-emitting injection Layer Layer emitting LayerLayer Electron-transport Layer Layer DBT3P-II:MoO_(x) TCTATCTA:Ir(tppr)₂(dpm) 2mDBTPDBq- 2mDBTPDBq- 2mDBTPDBq-II Bphen LiF 4:2 10nm 1:0.1 II:PCBA1BP:Ir(tBuppm)₂(acac) II:Ir(tBuppm)₂(acac) 10 nm 20 nm 1nm 40 nm 10 nm 0.8:0.2:0.05 0.8:0.05 5 nm 20 nm

The light-emitting element 3 was sealed using a glass substrate in aglove box containing a nitrogen atmosphere so as not to be exposed tothe air (specifically, a sealing material was applied onto an outer edgeof the element and heat treatment was performed at 80° C. for 1 hour atthe time of sealing).

In the light-emitting element 3, as in the light-emitting element 1,Ir(tBuppm)₂(acac) and Ir(tppr)₂(dpm) were used as the firstphosphorescent compound 113Da and the second phosphorescent compound113Db, respectively. Thus, the relation between the PL spectrum ofIr(tBuppm)₂(acac) and ε(λ)λ⁴ of Ir(tppr)₂(dpm) is the same as in thecase of the light-emitting element 1, and the description thereof is notrepeated. The description made with reference to FIGS. 20A and 20B inExample 1 is to be referred to. Accordingly, it is suggested that energytransfer efficiently occurs between the first phosphorescent compound113Da and the second phosphorescent compound 113Db in the light-emittingelement 3.

Element characteristics of the light-emitting element were measured.Note that the measurement was carried out at room temperature (in anatmosphere kept at 25° C.).

FIG. 30 shows luminance-current efficiency characteristics of thelight-emitting element 3. FIG. 31 shows voltage-luminancecharacteristics thereof. FIG. 32 shows luminance-external quantumefficiency characteristics thereof. FIG. 33 shows luminance-powerefficiency characteristics thereof.

From the above, the light-emitting element 3 turned out to haveexcellent element characteristics. In particular, as can be seen fromFIG. 30, FIG. 32, and FIG. 33, the light-emitting element has extremelyhigh emission efficiency and had a high external quantum efficiency notless than 20% at around a practical luminance (1000 cd/m²). In addition,the current efficiency was around 60 cd/A and the power efficiency wasaround 70 lm/W, which are excellent values.

FIG. 34 shows an emission spectrum of the light-emitting element 3 whichwas obtained when a current of 0.1 mA was made to flow in thelight-emitting element 3. FIG. 34 indicates that the light-emittingelement 3 shows an emission spectrum including light with a wavelengthin a green wavelength range which originates frombis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)) and light with a wavelength in a redwavelength range which originates frombis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: Ir(tppr)₂(dpm)) in a good balance.

From the above, it was shown that the light-emitting element 3corresponding to one embodiment of the present invention has highemission efficiency and provides lights from two kinds of emissioncenter substances in a good balance, although the host material which isdifferent from that of the light-emitting element 1 or 2 is used.

Example 4

In this example, a method for fabricating a light-emitting element whichcorresponds to one embodiment of the present invention described inEmbodiment 1 and Embodiment 2 and the characteristics thereof aredescribed. Structural formulae of organic compounds used in this exampleare shown below.

Next, a method for fabricating the light-emitting elements (alight-emitting element 4 and a light-emitting element 5) in this exampleis described below.

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness thereof was 110 nm and theelectrode area was 2 mm×2 mm. Here, the first electrode 101 is anelectrode that functions as an anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for about 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. The pressure in the vacuum evaporation apparatus was reducedto about 10⁻⁴ Pa. After that, over the first electrode 101,4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP)represented by Structural Formula (ii) and molybdenum(VI) oxide weredeposited by co-evaporation by an evaporation method using resistanceheating, so that the hole-injection layer 111 was formed. The thicknessof the hole-injection layer 111 was set to 33.3 nm, and the weight ratioof BPAFLP to molybdenum oxide was adjusted to 1:0.5. Note that theco-evaporation method refers to an evaporation method in whichevaporation is carried out from a plurality of evaporation sources atthe same time in one treatment chamber.

Next, a film of BPAFLP was formed to a thickness of 20 nm over thehole-injection layer 111 to form the hole-transport layer 112.

Further, over the hole-transport layer 112,2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II) represented by Structural Formula (iii),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP) represented by Structural Formula (Iv), andbis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)) represented by Structural Formula (v)were deposited by co-evaporation to a thickness of 20 nm with a massratio of 2mDBTPDBq-II to PCBA1BP and Ir(tBuppm)₂(acac) being0.8:0.2:0.06, so that the first light-emitting layer 113 a was formed;then, 2mDBTPDBq-II andbis{4,6-dimethyl-2-[3-(3,5-dimethylphenyl)-5-phenyl-2-pyrazinyl-κN]phenyl-κC}(2,6-dimethyl-3,5-heptanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(dmdppr-P)₂(dibm)]) represented by Structural Formula(xii) were deposited by co-evaporation to a thickness of 20 nm with amass ratio of 2mDBTPDBq-II to [Ir(dmdppr-P)₂(dibm)] being 1:0.06, sothat the second light-emitting layer 113 b was formed. Note that2mDBTPDBq-II, which is a host material, and PCBA1BP form an exciplex.

Then, the electron-transport layer 114 was formed over thelight-emitting layer 113 in such a way that a 15-nm-thick film of2mDBTPDBq-II was formed and a 15-nm-thick film of bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (vii) wasformed.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm, sothat the electron-injection layer 115 was formed.

Lastly, aluminum was deposited by evaporation to a thickness of 200 nmto form the second electrode 102 functioning as a cathode. Thus, thelight-emitting element 4 in this example was fabricated.

The light-emitting element 5 was fabricated in the same way as thelight-emitting element 4 except that the first light-emitting layer 113a contained, instead of [Ir(tBuppm)₂(acac)],tris[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC]iridium(III)(abbreviation: [Ir(tBuppm)₃)]) represented by Structural Formula (xiii).

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Tables 4 and 5 respectively show element structures of thelight-emitting elements 4 and 5 obtained as described above.

TABLE 4 Light-emitting Layer Hole-injection Hole-transport First Light-Second Light-emitting Electron-transport Electron- Layer Layer emittingLayer Layer Layer injection Layer BPAFLP:MoO_(x) BPAFLP2mDBTPDBq-II:PCBA1BP:Ir(tBuppm)₂(acac) 2mDBTPDBq- 2mDBTPDBq- Bphen LiF4:2 20 nm 0.8:0.2:0.06 II:Ir(dmdppr-P)₂(dibm) II 15 nm 1 nm 33 nm 20 nm1:0.06 15 nm 20 nm

TABLE 5 Light-emitting Layer Hole-injection Hole-transport First Light-Second Light-emitting Electron-injection Layer Layer emitting LayerLayer Electron-transport Layer Layer BPAFLP:MoO_(x) BPAFLP2mDBTPDBq-II:PCBA1BP:Ir(tBuppm)₃ 2mDBTPDBq- 2mDBTPDBq-II Bphen LiF 4:220 nm 0.8:0.2:0.06 II:Ir(dmdppr-P)₂(dibm) 15 nm 15 nm 1 nm 33 nm 20 nm1:0.06 20 nm

The light-emitting elements 4 and 5 were sealed using a glass substratein a glove box containing a nitrogen atmosphere so as not to be exposedto the air (specifically, a sealing material was applied onto an outeredge of the element and heat treatment was performed at 80° C. for 1hour at the time of sealing).

In the light-emitting element 4, Ir(tBuppm)₂(acac) and[Ir(dmdppr-P)₂(dibm)] were used as the first phosphorescent compound113Da and the second phosphorescent compound 113Db, respectively. Here,a relation between a PL spectrum of Ir(tBuppm)₂(acac) and ε(λ)λ⁴ of[Ir(dmdppr-P)₂(dibm)] is described. Note that λ denotes a wavelength andε(λ) denotes a molar absorption coefficient.

FIG. 45A shows graphs of the molar absorption coefficient ε(λ) andε(λ)λ⁴ of [Ir(dmdppr-P)₂(dibm)], which is the second phosphorescentcompound 113Db in the light-emitting element 4. While the molarabsorption coefficient ε(λ) does not have a noticeable peak in a regionon a longer wavelength side, the graph of ε(λ)λ⁴ has a peak including alocal maximum value at 509 nm and shoulders at around 550 nm and 605 nm.This peak shows triplet MLCT absorption of [Ir(dmdppr-P)₂(dibm)]. Whenthis peak has an overlap with an emission peak of the firstphosphorescent compound 113Da, energy transfer efficiency can be largelyincreased.

FIG. 45B shows the PL spectrum F(λ) of Ir(tBuppm)₂(acac) which is thefirst phosphorescent compound 113Da in the light-emitting element 4 andthe graph of ε(λ)λ⁴ of [Ir(dmdppr-P)₂(dibm)] which is the secondphosphorescent compound 113Db. As seen from the graph, a band having apeak of the PL spectrum F(λ) of Ir(tBuppm)₂(acac) largely overlaps withthe band having the longest-wavelength-side peak of ε(λ)λ⁴ of[Ir(dmdppr-P)₂(dibm)], which indicates that the combination enablesextremely efficient energy transfer. Further, Ir(tBuppm)₂(acac) which isthe first phosphorescent compound 113Da has an emission peak at 546 nm,and the spectrum showing ε(λ)λ⁴ of [Ir(dmdppr-P)₂(dibm)] which is thesecond phosphorescent compound 113Db has a longer-wavelength-side localmaximum at 509 nm, so that the difference is 37 nm. The wavelengths 546nm and 509 nm correspond to 2.27 eV and 2.44 eV, respectively, so thatthe difference is 0.17 eV, which is less than 0.2 eV; thus, thepositions of the peaks also suggest occurrence of efficient energytransfer. Note that although the longer-wavelength-side peak (peak C) inthe spectrum showing ε(λ)λ⁴ of [Ir(dmdppr-P)₂(dibm)] hardly overlapswith the spectrum F(λ) of Ir(tBuppm)₂(acac), the band including the peakC in the spectrum showing ε(λ)λ⁴ of [Ir(dmdppr-P)₂(dibm)] has a broadshape on the longer wavelength side, and on the longer wavelength side,the spectrum has a large overlap with the emission spectrum F(λ) ofIr(tBuppm)₂(acac). Accordingly, extremely efficient energy transfer isachieved.

In the light-emitting element 4, 2mDBTPDBq-II, which is the first hostmaterial, and PCBA1BP, which is the first organic compound, form anexciplex, so that energy is efficiently transferred toIr(tBuppm)₂(acac), which is the first phosphorescent compound 113Da. Therelation is similar to that in the light-emitting element 1 anddescribed in detail in Example 1; thus, the description is not repeated.The corresponding description in Example 1 is to be referred to.

FIG. 46 shows the PL spectrum F(λ) of the exciplex, the PL spectrum F(λ)of Ir(tBuppm)₂(acac), a PL spectrum F(λ) of [Ir(dmdppr-P)₂(dibm)],ε(λ)λ⁴ of Ir(tBuppm)₂(acac), and ε(λ)λ⁴ of [Ir(dmdppr-P)₂(dibm)]. It canbe found that energy can be transferred stepwise first from the exciplexto Ir(tBuppm)₂(acac) by utilizing the overlap between the PL spectrum ofthe exciplex and ε(λ)λ⁴ of Ir(tBuppm)₂(acac) (around the peak A), andthen from Ir(tBuppm)₂(acac) to [Ir(dmdppr-P)₂(dibm)] by utilizing theoverlap between the PL spectrum of Ir(tBuppm)₂(acac) and ε(λ)λ⁴ of[Ir(dmdppr-P)₂(dibm)] (around a range from the peak C to 650 nm). Notethat direct energy transfer from the exciplex to [Ir(dmdppr-P)₂(dibm)]which is the second phosphorescent compound is also possible. The reasonfor this is that, as can be seen from FIG. 46, ε(λ)λ⁴ of[Ir(dmdppr-P)₂(dibm)] also overlaps with the PL spectrum F(λ) of theexciplex in the triplet MLCT absorption band (around the peak C) of[Ir(dmdppr-P)₂(dibm)].

In the light-emitting element 5, Ir(tBuppm)₃ and [Ir(dmdppr-P)₂(dibm)]were used as the first phosphorescent compound 113Da and the secondphosphorescent compound 113Db, respectively. Here, a relation between aPL spectrum of Ir(tBuppm)₃ and ε(λ)λ⁴ of [Ir(dmdppr-P)₂(dibm)] isdescribed. Note that λ denotes a wavelength and ε(λ) denotes a molarabsorption coefficient.

FIG. 47A shows graphs of the molar absorption coefficient ε(λ) andε(λ)λ⁴ of [Ir(dmdppr-P)₂(dibm)], which is the second phosphorescentcompound 113Db in the light-emitting element 5. While the molarabsorption coefficient ε(λ) does not have a noticeable peak in a regionon a longer wavelength side, the graph of ε(λ)λ⁴ has a peak including alocal maximum value at 509 nm and shoulders at around 550 nm and 605 nm.This peak shows triplet MLCT absorption of [Ir(dmdppr-P)₂(dibm)]. Whenthis peak has an overlap with an emission peak of the firstphosphorescent compound 113Da, energy transfer efficiency can be largelyincreased.

FIG. 47B shows the PL spectrum F(λ) of Ir(tBuppm)₃ which is the firstphosphorescent compound 113Da in the light-emitting element 5 and thegraph of ε(λ)λ⁴ of [Ir(dmdppr-P)₂(dibm)] which is the secondphosphorescent compound 113Db. As seen from the graph, a band having apeak of the PL spectrum F(λ) of Ir(tBuppm)₃ largely overlaps with theband having the longest-wavelength-side peak of ε(λ)λ⁴ of[Ir(dmdppr-P)₂(dibm)], which indicates that the combination enablesextremely efficient energy transfer. Further, Ir(tBuppm)₃ which is thefirst phosphorescent compound 113Da has an emission peak at 540 nm, andthe spectrum showing ε(λ)λ⁴ of [Ir(dmdppr-P)₂(dibm)] which is the secondphosphorescent compound 113Db has a longer-wavelength-side local maximumat 509 nm, so that the difference is 31 nm. The wavelengths 540 nm and509 nm correspond to 2.30 eV and 2.44 eV, respectively, so that thedifference is 0.14 eV, which is less than 0.2 eV; thus, the positions ofthe peaks also suggest occurrence of efficient energy transfer. Notethat although the longer-wavelength-side local maximum (peak C) in thespectrum showing ε(λ)λ⁴ of [Ir(dmdppr-P)₂(dibm)] hardly overlaps withthe spectrum F(λ) of Ir(tBuppm)₃, the band including the peak C in thespectrum showing ε(λ)λ⁴ of [Ir(dmdppr-P)₂(dibm)] has a broad shape onthe longer wavelength side, and on the longer wavelength side, thespectrum has a large overlap with the emission spectrum F(λ) ofIr(tBuppm)₃. Accordingly, extremely efficient energy transfer isachieved.

Next, FIG. 48A shows graphs of the molar absorption coefficient ε(λ) andε(λ)λ⁴ of Ir(tBuppm)₃, which is the first phosphorescent compound 113Dain the light-emitting element 5. The graph of ε(λ)λ⁴ has peaks with highintensities at 409 nm and 465 nm and a peak including a shoulder at 494nm. This peak shows triplet MLCT absorption of Ir(tBuppm)₃. When thispeak has an overlap with an emission peak of an energy donor, energytransfer efficiency can be largely increased.

Here, in the light-emitting element 5 in this example, 2mDBTPDBq-IIwhich is the first host material and PCBA1BP which is the first organiccompound form the exciplex 113Ec, and energy is transferred from theexciplex 113Ec to the first phosphorescent compound 113Da. FIG. 23 showsPL spectra of 2mDBTPDBq-II, PCBA1BP, and a mixed film thereof (a massratio of 2mDBTPDBq-II to PCBA1BP is 0.8:0.2), and it can be found that2mDBTPDBq-II and PCBA1BP which is the first organic compound formed theexciplex 113Ec. FIG. 48B shows a PL spectrum F(λ) of the exciplex andthe graph of ε(λ)λ⁴ of Ir(tBuppm)₃ which is the first phosphorescentcompound 113Da. As seen from the graph, part of a wavelength range inwhich a band having a peak of the PL spectrum F(λ) of the exciplex hashalf of the intensity of the peak overlaps with part of a wavelengthrange in which a band with the longest-wavelength-side peak of ε(λ)λ⁴ ofIr(tBuppm)₃ has half of the intensity of the peak, which indicates thatthe combination enables efficient energy transfer.

FIG. 49 shows the PL spectrum F(λ) of the exciplex, the PL spectrum F(λ)of Ir(tBuppm)₃, a PL spectrum F(λ) of [Ir(dmdppr-P)₂(dibm)], ε(λ)λ⁴ ofIr(tBuppm)₃, and ε(λ)λ⁴ of [Ir(dmdppr-P)₂(dibm)]. It can be found thatenergy can be transferred stepwise first from the exciplex toIr(tBuppm)₃ by utilizing the overlap between the PL spectrum of theexciplex and ε(λ)λ⁴ of Ir(tBuppm)₃ (around the peak A), and then fromIr(tBuppm)₃ to [Ir(dmdppr-P)₂(dibm)] by utilizing the overlap betweenthe PL spectrum of Ir(tBuppm)₃ and ε(λ)λ⁴ of [Ir(dmdppr-P)₂(dibm)](around a range from the peak C to 650 nm). Note that direct energytransfer from the exciplex to [Ir(dmdppr-P)₂(dibm)] which is the secondphosphorescent compound is also possible. The reason for this is that,as can be seen from FIG. 49, ε(λ)λ⁴ of [Ir(dmdppr-P)₂(dibm)] alsooverlaps with the PL spectrum F(λ) of the exciplex in the triplet MLCTabsorption band (around the peak C) of [Ir(dmdppr-P)₂(dibm)].

Element characteristics of these light-emitting elements were measured.Note that the measurement was carried out at room temperature (in anatmosphere kept at 25° C.).

FIG. 35 shows luminance-current efficiency characteristics of thelight-emitting element 4. FIG. 36 shows voltage-luminancecharacteristics thereof. FIG. 37 shows luminance-external quantumefficiency characteristics thereof. FIG. 38 shows luminance-powerefficiency characteristics thereof.

From the above, the light-emitting element 4 turned out to haveexcellent element characteristics. In particular, as can be seen fromFIG. 35, FIG. 37, and FIG. 38, the light-emitting element has extremelyhigh emission efficiency and had a high external quantum efficiency notless than 20% at around a practical luminance (1000 cd/m²). In addition,the current efficiency was around 50 cd/A and the power efficiency wasaround 50 lm/W, which are excellent values.

FIG. 39 shows an emission spectrum of the light-emitting element 4 whichwas obtained when a current of 0.1 mA was made to flow in thelight-emitting element 4. In FIG. 39, the horizontal axis indicates awavelength (nm) and the vertical axis indicates light emission intensity(arbitrary unit). FIG. 39 indicates that the light-emitting element 4shows an emission spectrum including light with a wavelength in a greenwavelength range which originates from [Ir(tBuppm)₂(acac)] and lightwith a wavelength in a red wavelength range which originates from[Ir(dmdppr-P)₂(dibm)] in a good balance.

FIG. 40 shows luminance-current efficiency characteristics of thelight-emitting element 5. FIG. 41 shows voltage-luminancecharacteristics thereof. FIG. 42 shows luminance-external quantumefficiency characteristics thereof. FIG. 43 shows luminance-powerefficiency characteristics thereof.

From the above, the light-emitting element 5 turned out to haveexcellent element characteristics. In particular, as can be seen fromFIG. 40, FIG. 42, and FIG. 43, the light-emitting element has extremelyhigh emission efficiency and had a high external quantum efficiency ofabout 25% at around a practical luminance (1000 cd/m²). In addition, thecurrent efficiency was around 65 cd/A and the power efficiency wasaround 70 lm/W, which are excellent values.

FIG. 44 shows an emission spectrum of the light-emitting element 5 whichwas obtained when a current of 0.1 mA was made to flow in thelight-emitting element 5. In FIG. 44, the horizontal axis indicates awavelength (nm) and the vertical axis indicates light emission intensity(arbitrary unit). FIG. 44 indicates that the light-emitting element 5shows an emission spectrum including light with a wavelength in a greenwavelength range which originates from [Ir(tBuppm)₂(acac)] and lightwith a wavelength in a red wavelength range which originates from[Ir(dmdppr-P)₂(dibm)] in a good balance.

From the above, it was shown that the light-emitting elements 4 and 5each corresponding to one embodiment of the present invention have highemission efficiency and provide lights from two kinds of emission centersubstances in a good balance.

Reference Example 1

A synthesis example of the organometallic complexbis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III) (abbreviation: [Ir(tBuppm)₂(acac)]), which is used in theabove embodiment, is described. The structure of [Ir(tBuppm)₂(acac)] isshown below.

Step 1: Synthesis of 4-tert-Butyl-6-phenylpyrimidine (abbreviation:HtBuppm)

First, 22.5 g of 4,4-dimethyl-1-phenylpentane-1,3-dione and 50 g offormamide were put into a recovery flask equipped with a reflux pipe,and the air in the flask was replaced with nitrogen. This reactioncontainer was heated, so that the reacted solution was refluxed for 5hours. After that, this solution was poured into an aqueous solution ofsodium hydroxide, and an organic layer was extracted withdichloromethane. The obtained organic layer was washed with water and asaturated aqueous solution of sodium chloride, and dried with magnesiumsulfate. The solution after drying was filtered. The solvent of thissolution was distilled off, and then the obtained residue was purifiedby silica gel column chromatography using hexane and ethyl acetate as adeveloping solvent in a volume ratio of 10:1, so that a pyrimidinederivative HtBuppm (colorless oily substance, yield of 14%) wasobtained. A synthesis scheme of Step 1 is shown below.

Step 2: Synthesis ofDi-μ-chloro-bis[bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)](abbreviation: [Ir(tBuppm)₂Cl]₂)

Next, 15 mL of 2-ethoxyethanol, 5 mL of water, 1.49 g of HtBuppmobtained in Step 1, and 1.04 g of iridium chloride hydrate (IrCl₃.H₂O)were put into a recovery flask equipped with a reflux pipe, and the airin the flask was replaced with argon. After that, irradiation withmicrowaves (2.45 GHz, 100 W) was performed for 1 hour to cause areaction. The solvent was distilled off, and then the obtained residuewas suction-filtered and washed with ethanol, so that a dinuclearcomplex [Ir(tBuppm)₂Cl]₂ (yellow green powder, yield of 73%) wasobtained. A synthesis scheme of Step 2 is shown below.

Step 3: Synthesis of(Acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)])

Further, 40 mL of 2-ethoxyethanol, 1.61 g of the dinuclear complex[Ir(tBuppm)₂Cl]₂ obtained in Step 2, 0.36 g of acetylacetone, and 1.27 gof sodium carbonate were put into a recovery flask equipped with areflux pipe, and the air in the flask was replaced with argon. Afterthat, irradiation with microwaves (2.45 GHz, 120 W) was performed for 60minutes to cause a reaction. The solvent was distilled off, and theobtained residue was suction-filtered with ethanol and washed with waterand ethanol. This solid was dissolved in dichloromethane, and themixture was filtered through a filter aid in which Celite (produced byWako Pure Chemical Industries, Ltd., Catalog No. 531-16855), alumina,and Celite were stacked in this order. The solvent was distilled off,and the obtained solid was recrystallized with a mixed solvent ofdichloromethane and hexane, so that the objective substance was obtainedas yellow powder (yield of 68%). A synthesis scheme of Step 3 is shownbelow.

An analysis result by nuclear magnetic resonance (¹H NMR) spectroscopyof the yellow powder obtained in Step 3 is described below. The resultrevealed that the organometallic complex Ir(tBuppm)₂(acac) was obtained.

¹H NMR. δ (CDCl₃): 1.50 (s, 18H), 1.79 (s, 6H), 5.26 (s, 1H), 6.33 (d,2H), 6.77 (t, 2H), 6.85 (t, 2H), 7.70 (d, 2H), 7.76 (s, 2H), 9.02 (s,2H).

Reference Example 2

In this reference example, a synthesis method ofbis{4,6-dimethyl-2-[3-(3,5-dimethylphenyl)-5-phenyl-2-pyrazinyl-κN]phenyl-κC}(2,6-dimethyl-3,5-heptanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(dmdppr-P)₂(dibm)]), the organometallic iridiumcomplex used in the example, is described. The structure of[Ir(dmdppr-P)₂(dibm)] (abbreviation) is shown below.

Step 1: Synthesis of 2,3-Bis(3,5-dimethylphenyl)pyrazine (abbreviation:Hdmdppr)

First, 5.00 g of 2,3-dichloropyrazine, 10.23 g of3,5-dimethylphenylboronic acid, 7.19 g of sodium carbonate, 0.29 g ofbis(triphenylphosphine)palladium(II) dichloride (abbreviation:Pd(PPh₃)₂Cl₂), 20 mL of water, and 20 mL of acetonitrile were put into arecovery flask equipped with a reflux pipe, and the air in the flask wasreplaced with argon. This reaction container was subjected toirradiation with microwaves (2.45 GHz, 100 W) for 60 minutes to beheated. Here, into the flask were further put 2.55 g of3,5-dimethylphenylboronic acid, 1.80 g of sodium carbonate, 0.070 g ofPd(PPh₃)₂Cl₂, 5 mL of water, and 5 mL of acetonitrile, and irradiationwith microwaves (2.45 GHz, 100 W) was performed again for 60 minutes sothat heating was performed.

Then, water was added to this solution and the organic layer wasextracted with dichloromethane. The obtained organic layer was washedwith a saturated aqueous solution of sodium hydrogen carbonate, water,and a saturated aqueous solution of sodium chloride, and was dried withmagnesium sulfate. After the drying, the solution was filtered. Thesolvent of this solution was distilled off, and the obtained residue waspurified by flash column chromatography using hexane and ethyl acetateas a developing solvent in a volume ratio of 5:1. The solvent wasdistilled off, and the obtained solid was purified by flash columnchromatography using dichloromethane and ethyl acetate as a developingsolvent in a volume ratio of 10:1, so that Hdmdppr (abbreviation), whichwas the pyrazine derivative to be produced, was obtained as a whitepowder in a yield of 44%. Note that the irradiation with microwaves wasperformed using a microwave synthesis system (Discover, manufactured byCEM Corporation). A synthesis scheme of Step 1 is shown in (a-1).

Step 2: Synthesis of 2,3-Bis(3,5-dimethylphenyl)-5-phenylpyrazine(abbreviation: Hdmdppr-P)

First, 4.28 g of Hdmdppr (abbreviation) obtained in Step 1 and 80 mL ofdry THF were put into a three-neck flask and the air in the flask wasreplaced with nitrogen. After the flask was cooled with ice, 9.5 mL ofphenyl lithium (1.9M solution of phenyl lithium in butyl ether) wasadded dropwise, and the mixture was stirred at room temperature for 23.5hours. The reacted solution was poured into water and the solution wassubjected to extraction with chloroform. The obtained organic layer waswashed with water and a saturated aqueous solution of sodium chloride,and dried with magnesium sulfate. Manganese oxide was added to theobtained mixture and the mixture was stirred for 30 minutes. Then, thesolution was filtered and the solvent was distilled off. The obtainedresidue was purified by silica gel column chromatography usingdichloromethane as a developing solvent, so that Hdmdppr-P(abbreviation), which was the pyrazine derivative to be produced, wasobtained as an orange oil in a yield of 26%. A synthesis scheme of Step2 is shown in (a-2).

Step 3: Synthesis ofDi-μ-chloro-tetrakis{4,6-dimethyl-2-[3-(3,5-dimethylphenyl)-5-phenyl-2-pyrazinyl-κN]phenyl-κC}diiridium(III)(abbreviation: [Ir(dmdppr-P)₂Cl]₂)

Next, into a recovery flask equipped with a reflux pipe were put 15 mLof 2-ethoxyethanol, 5 mL of water, 1.40 g of Hdmdppr-P (abbreviation)obtained in Step 2, and 0.51 g of iridium chloride hydrate (IrCl₃.H₂O)(produced by Sigma-Aldrich Corporation), and the air in the flask wasreplaced with argon. After that, irradiation with microwaves (2.45 GHz,100 W) was performed for 1 hour to cause a reaction. The solvent wasdistilled off, and then the obtained residue was suction-filtered andwashed with ethanol to give [Ir(dmdppr-P)₂Cl]₂ (abbreviation) that is adinuclear complex as a reddish brown powder in a yield of 58%. Asynthesis scheme of Step 3 is shown in (a-3).

Step 4: Synthesis of Bis{4,6-dimethyl-2-[3-(3,5-dimethylphenyl)-5-phenyl-2-pyrazinyl-κN]phenyl-κC}(2,6-dimethyl-3,5-heptanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(dmdppr-P)₂(dibm)])

Further, into a recovery flask equipped with a reflux pipe were put 30mL of 2-ethoxyethanol, 0.94 g of [Ir(dmdppr-P)₂Cl]₂ that is thedinuclear complex obtained in Step 3, 0.23 g of diisobutyrylmethane(abbreviation: Hdibm), and 0.52 g of sodium carbonate, and the air inthe flask was replaced with argon. After that, the mixture was heated byirradiation with microwaves (2.45 GHz, 120 W) for 60 minutes. Thesolvent was distilled off, and the obtained residue was suction-filteredwith ethanol. The obtained solid was washed with water and ethanol andrecrystallization was carried out with a mixed solvent ofdichloromethane and ethanol, so that [Ir(dmdppr-P)₂(dibm)](abbreviation), the organometallic complex in one embodiment of thepresent invention, was obtained as a dark red powder in a yield of 75%.A synthesis scheme of Step 4 is shown in (a-4).

An analysis result by nuclear magnetic resonance (¹H-NMR) spectroscopyof the dark red powder obtained by the above-described synthesis methodis described below. These results revealed that the organometalliccomplex [Ir(dmdppr-P)₂(dibm)] (abbreviation) was obtained in thissynthesis example.

¹H-NMR. δ (CDCl₃): 0.79 (d, 6H), 0.96 (d, 6H), 1.41 (s, 6H), 1.96 (s,6H), 2.24-2.28 (m, 2H), 2.41 (s, 12H), 5.08 (s, 1H), 6.46 (s, 2H), 6.82(s, 2H), 7.18 (s, 2H), 7.39-7.50 (m, 10H), 8.03 (d, 4H), 8.76 (s, 2H).

REFERENCE NUMERALS

10: electrode, 11: electrode, 101: first electrode, 102: secondelectrode, 103: EL layer, 111: hole-injection layer, 112: hole-transportlayer, 113: light-emitting layer, 113 a: first light-emitting layer,113Da: first phosphorescent compound, 113Ha: first host material, 113 b:second light-emitting layer, 113Db: second phosphorescent compound,113Hb: second host material, 113A: first organic compound, 113Ec:exciplex, 114: electron-transport layer, 115: electron-injection layer,400: substrate, 401: first electrode, 402: auxiliary electrode, 403: ELlayer, 404: second electrode, 405: sealing material, 406: sealingmaterial, 407: sealing substrate, 412: pad, 420: IC chip, 601: drivercircuit portion (source line driver circuit), 602: pixel portion, 603:driver circuit portion (gate line driver circuit), 604: sealingsubstrate, 605: sealing material, 607: space, 608: wiring, 609: FPC(flexible printed circuit), 610: element substrate, 611: switching TFT,612: current controlling TFT, 613: first electrode, 614: insulator, 616:EL layer, 617: second electrode, 618: light-emitting element, 623:n-channel TFT, 624: p-channel TFT, 625: drying agent, 901: housing, 902:liquid crystal layer, 903: backlight unit, 904: housing, 905: driver IC,906: terminal, 951: substrate, 952: electrode, 953: insulating layer,954: partition layer, 955: EL layer, 956: electrode, 1001: substrate,1002: base insulating film, 1003: gate insulating film, 1006: gateelectrode, 1007: gate electrode, 1008: gate electrode, 1020: firstinterlayer insulating film, 1021: second interlayer insulating film,1022: electrode, 1024W: first electrode of a light-emitting element,1024R: first electrode of a light-emitting element, 1024G: firstelectrode of a light-emitting element, 1024B: first electrode of alight-emitting element, 1025: partition wall, 1028: EL layer, 1029:second electrode of a light-emitting element, 1031: sealing substrate,1032: sealant, 1033: transparent base material, 1034R: red coloringlayer, 1034G: green coloring layer, 1034B: blue coloring layer, 1035:black layer (black matrix), 1036: overcoat layer, 1037: third interlayerinsulating film, 1040: pixel portion, 1041: driver circuit portion,1042: peripheral portion, 1044W: white light-emitting region, 1044R: redlight-emitting region, 1044B: blue light-emitting region, 1044G: greenlight-emitting region, 2001: housing, 2002: light source, 3001: lightingdevice, 3002: display device, 5000: display, 5001: display, 5002:display, 5003: display, 5004: display, 5005: display, 7101: housing,7103: display portion, 7105: stand, 7107: display portion, 7109:operation key, 7110: remote controller, 7201: main body, 7202: housing,7203: display portion, 7204: keyboard, 7205: external connection port,7206: pointing device, 7210: second display portion, 7301: housing,7302: housing, 7303: joint portion, 7304: display portion, 7305: displayportion, 7306: speaker portion, 7307: recording medium insertionportion, 7308: LED lamp, 7309: operation key, 7310: connection terminal,7311: sensor, 7400: mobile phone, 7401: housing, 7402: display portion,7403: operation button, 7404: external connection port, 7405: speaker,7406: microphone, 9033: clasp, 9034: switch, 9035: power switch, 9036:switch, 9038: operation switch, 9630: housing, 9631: display portion,9631 a: display portion, 9631 b: display portion, 9632 a: touchscreenregion, 9632 b: touchscreen region, 9633: solar cell, 9634: charge anddischarge control circuit, 9635: battery, 9636: DC-to-DC converter,9637: operation key, 9638: converter, and 9639: button.

This application is based on Japanese Patent Application serial no.2012-096808 filed with Japan Patent Office on Apr. 20, 2012, andJapanese Patent Application serial no. 2013-052791 filed with JapanPatent Office on Mar. 15, 2013, the entire contents of which are herebyincorporated by reference.

The invention claimed is:
 1. A light-emitting element comprising: afirst electrode; a first light-emitting layer over the first electrodecomprising: a first phosphorescent compound; and a first host material;a second light-emitting layer over the first light-emitting layercomprising: a second phosphorescent compound; and a second hostmaterial; a second electrode over the second light-emitting layer,wherein light emitted from the second phosphorescent compound has alonger wavelength than light emitted from the first phosphorescentcompound, wherein a longest-wavelength-side peak of a function ε(λ)λ⁴ ofthe second phosphorescent compound overlaps with a phosphorescencespectrum F(λ) of the first phosphorescent compound, wherein λ denotes awavelength, and wherein ε(λ) denotes a molar absorption coefficient atthe wavelength λ.
 2. The light-emitting element according to claim 1,wherein the first light-emitting layer further contains a first organiccompound, wherein the first host material and the first organic compoundform an exciplex, and wherein the light emitted from the firstphosphorescent compound has a longer wavelength than light emitted fromthe exciplex.
 3. The light-emitting element according to claim 2,wherein an emission spectrum of the exciplex overlaps with thelongest-wavelength-side peak of the function ε(λ)λ⁴ of the firstphosphorescent compound.
 4. The light-emitting element according toclaim 1, wherein the first phosphorescent compound has a phosphorescencepeak in a range of 500 nm to 600 nm, and wherein the secondphosphorescent compound has a phosphorescence peak in a range of 600 nmto 700 nm.
 5. The light-emitting element according to claim 1, wherein arecombination region of an electron and a hole is the firstlight-emitting layer.
 6. The light-emitting element according to claim1, wherein the first light-emitting layer is positioned closer to ananode than the second light-emitting layer, and wherein anelectron-transport property is higher than a hole-transport property inthe second light-emitting layer.
 7. The light-emitting element accordingto claim 1, wherein the first light-emitting layer is positioned closerto an anode than the second light-emitting layer, and wherein both thefirst host material and the second host material have anelectron-transport property.
 8. The light-emitting element according toclaim 1, wherein the first light-emitting layer is positioned closer toa cathode than the second light-emitting layer, and wherein ahole-transport property is higher than an electron-transport property inthe second light-emitting layer.
 9. The light-emitting element accordingto claim 1, wherein the first light-emitting layer is positioned closerto a cathode than the second light-emitting layer, and wherein both thefirst host material and the second host material have a hole-transportproperty.
 10. The light-emitting element according to claim 1, whereinthe first light-emitting layer and the second light-emitting layer arein contact with each other.
 11. A lighting device comprising thelight-emitting element according to claim
 1. 12. A light-emitting devicecomprising: the light-emitting element according to claim 1; and a meansfor controlling the light-emitting element.
 13. A display devicecomprising: the light-emitting element according to claim 1 in a displayportion; and a means for controlling the light-emitting element.
 14. Anelectronic device comprising the light-emitting element according toclaim 1.